D&D Monsters: Ettins

The word "giant" entered the English language in the Middle Ages, coming from the Latin and Greek gigas via Norman French. Prior to this time, the English word for what we would now call a giant was "ettin", which is related to the Norse jotunn. This older word fell out of favour, although it was used by both Lewis and Tolkien as the names for giant or troll inhabited regions in their fictional worlds: Ettinsmoor, north of Narnia, and the Ettenmoors, northeast of the Shire. (For that matter, Tolkein also used the alternative form of the word, "ent", as the name for a rather different race of gigantic beings).

In any event, "ettin" is simply another, and older, word for "giant". In D&D, however, it specifically refers to a kind of two-headed giant; this is original to the game, but has been adopted by some other writers and computer game designers since.

D&D Monsters: Frost and Fire Giants

Both frost and fire giants, as portrayed in D&D, are loosely based on the jötunn of Norse myth. In the original myths, this was a rather vague term referring to a race of mostly (but not always) gigantic beings originally created by the primordial being Ymir. Ymir himself was certainly gigantic, since the sky is said to be the inside of his blue-coloured skull, and he was also described in the Prose Edda as "ice cold". Thus, while frost giants aren't a specific thing in Norse myth, it's clear where the idea comes from. Fire giants, apparently also descended from Ymir in the original myth, are the inhabitants of the fiery realm of Muspelheim, led by Surtur, whose primary function in the myths is to destroy the world at the conclusion of Ragnarok.

Both frost and fire giants appear in a range of fictional sources, of which perhaps the most obvious are the Mighty Thor comics. These predate D&D by over ten years, although it's likely that their appearance in the game is an independent creation from the same mythic origin, since they're quite clearly different from the comicbook versions. In D&D, they are the third and fourth rungs on the six-step ladder of increasing giant power and strength and are portrayed as more technologically advanced than the smaller hill and stone giants.