So, the name...

"There were once two men who went up into the mountains to gather edible moss. One night they were sharing a tent, and one was asleep but the other awake. The one who was awake saw the one who was asleep go creeping out; he got up and followed him, but however hard he ran he could not catch up with him. The sleeping man was heading straight up the mountain towards the glaciers, and the other saw where a huge giantess was sitting up there on a spur of the glacier. What she was doing was this: she would stretch out her arms with her hands crossed and then draw them in again to her breast, and in this way she was magically drawing the man towards her. The man ran straight into her arms, and she then ran off with him.
"A year later, some people from the man's district were gathering moss at the same place; he came there to meet them, and he was so short-spoken and surly that one could hardly get a word out of him. They asked him who he believed in, and he said he believed in God. The following year he came to the moss-gatherers again, and by then he looked so like a troll that he struck terror into them. However, he was asked again who he believed in, but he made no reply. This time he stayed a shorter time with them than before. The third year, he came again; by then he had turned into an absolute troll, and a very ugly-looking one too. Yet someone plucked up the courage to ask him who he believed in, but he said he believed in 'Trunt, Trunt, and the trolls in the fells' - and then he disappeared. After this he was never seen again, but for some years afterwards men did not dare go looking for moss in that place."
Jacqueline Simpson, Icelandic Folktales and Legends (1972)