Etymology

The meaning of the word troll is uncertain. It might have had the originally meaning of supernatural or Magic with an overlay of malignant and perilous. Another likely suggestion is that it means "someone who behaves violently".

In old Swedish law, trolleri was a particular kind of magic intended to do harm. It should be noted that North Germanic terms such as trolldom (witchcraft) and trolla/trylle (perform magic tricks) does not imply any connection with the mythical beings. Moreover, in the sources for Norse mythology, troll can signify any uncanny being, including but not restricted to the Norse giants. The ambiguous original sense of the word troll appears to have lived on for some time after the Old Norse literature was documented. This can be seen in terms such as sjötrollet (the sea troll) as a synonym for havsmannen (the sea man) – a protective spirit of the sea and a sort of male counterpart to the female sjörå (see huldra).

Opinions vary as to whether or not trolls were thoroughly bad or not, but often they treated people as they were treated. Trolls could cause great harm if vindictive or playful, though, and regardless of other things they were always heathen. Trolls were also great thieves, and liked to steal from the food that the farmers had stored. They could enter the homes invisibly during feasts and eat from the plates so that there was not enough food, or spoil the making of beer and bread so that it failed or did not end up plentiful enough.

The trolls sometimes abducted people to live as slaves or at least prisoners among them. These poor souls were known as bergtagna ("those taken to/by the mountain"), which also is the Scandinavian word for having been spirited away. To be bergtagen does not only refer to the disappearance of the person, but also that upon returning, he or she has been struck with insanity or apathy caused by the trolls. Anyone could be taken by the trolls, even cattle, but at the greatest risk were women who had given birth but not yet been taken back to the church. Occasionally, the trolls would even steal a new-born baby, leaving their own offspring – a (bort)byting ("changeling") – in return.

More often than not, though, the trolls kept themselves invisible, and then they could travel on the winds, such as the wind-troll Ysätters-Kajsa, or sneak into human homes. Sometimes you could only hear them speak, shout and make noise, or the sound of their cattle. Similarly, if you were out in the forest and smelled food cooking, you knew you were near a troll dwelling. The trolls were also great shapeshifters, taking shapes of objects like fallen logs or animals like cats and dogs. A fairly frequent notion is that the trolls liked to appear as rolling balls of yarn.

To ward off the trolls you could always trust in Christianity: Church bells, a cross or even words like "Jesus" or "Christ" would work against them. Like other Scandinavian folklore creatures they also feared steel. Apart from that they were hunted by Thor, one of the last remnants of the old Norse mythology, who threw Mjolnir, his hammer, causing lightning bolts to kill them. Though Mjolnir was supposed to return to Thor after throwing, these hammers could later be found in the earth (actually Stone Age axes) and be used as protective talismans.

In Scandinavian fairy tales trolls sometimes turn to stone if exposed to sunlight. Some have attributed this aspect of the myth with pareidolia found in naturally eroded rock outcrops.

Like many other species in Scandinavian folklore, they were said to reside in underground complexes, accessible from underneath large boulders in the forests or in the mountains. These boulders could be raised upon pillars of gold.

In some Norwegian accounts, such as the middle age ballade Åsmund Frægdegjevar, the trolls live in a far northern land called Trollebotten – the concept and location of which seems to coincide with the Old Norse Jötunheimr. There are many places in Scandinavia that are named after trolls, such as the Swedish town Trollhättan (Troll's bonnet) and the legendary mountain Trollkyrka (Troll church).

Legends from the Middle Ages and earlier feature a kind of trolls of horrifying dimensions. This might reflect a past view of trolls as distinctly bad creatures that would soften in later folklore, or just be another example of fantastic tales demanding fantastic dimensions.

Many of the fairytales featuring trolls were written in the late 19th century to early 20th century, reflecting the romanticism of the time, and published in fairytale collections like Tomtar och Troll. These tales, and illustrations by artists like John Bauer and Theodor Kittelsen, would come to form the ideas most people have of trolls today.

Asbjørnsen and Moe's collection feature a number of traditional fairy tales where trolls hold princesses captive, such as The Three Princesses of Whiteland, Soria Moria Castle, and Dapplegrim, and two where trolls invade homes on Christmas Eve to make merry, Tatterhood and The Cat on the Dovrefell. Female trolls may conspire to force the prince to marry their daughters, as in East of the Sun and West of the Moon, or practice witchcraft, as in The Witch in the Stone Boat, where a troll usurps a queen's place, or The Twelve Wild Ducks, where she turns twelve princes into wild ducks. In other tales, the hero matches wits with the troll: Boots and the Troll, and Boots Who Ate a Match With the Troll.

Theories about origin and existence

In the genre of paleofiction, the distinguished Finnish paleontologist Björn Kurtén has entertained the theory (e.g. in Dance of the Tiger) that trolls are a distant memory of an encounter with Neanderthals by our Cro-Magnon ancestors some 40,000 years ago during their migration into northern Europe. Spanish paleoanthropologist Juan Luis Arsuaga provides evidence for these types of encounters in his book, The Neanderthal's Necklace. The theory that Neanderthals and Cro-Magnons occupied the same area of Europe at the same time in history has been theorized based on fossil evidence. Other researchers believe that they just refer to neighboring tribes. The problem with this theory is that neither Neanderthals or Cro-Magnons existed in this part of Europe during the ice-age. Most of Scandinavia was covered by a large glacier and the area was not occupied until much later.

A more plausible explanation for the troll myth, is that the trolls represent the remains of the forefather-cult which was ubiquitous in Scandinavia until the introduction of Christianity in the 10th and 11th centuries. In this cult the forefathers were worshipped in sacred groves, by altars or by gravemounds. One of the customs associated with this practice was to sit on top of a gravemound at night, possibly in order to make contact with the deceased. With the introduction of Christianity however, the religious elite sought to demonize the pagan cult, and denounced the forefathers as evil. For instance, according to Magnus Håkonsen's laws from 1276 it is illegal to attempt to wake the "mound-dwellers". It is in these laws that the word troll appears for the first time, denoting something heathen and generally unfavourable.

This fits with the trolls in Norse sagas who are often the restless dead, to be wrestled with or otherwise laid to rest.