Ettins in the D&D universe are described as giants which have two heads, and this has carried over to many different games that draw from the D&D lore, for example Dwarf Fortress and many roguelikes.
However, the original etymology for the word doesn't imply any dicephalous nature of the giants, and I was unable to find an earlier citation that described ettins with two heads.
Was the description of ettins having two heads based on Norse (or any other) mythology?