Yours, Etc.: Origins and Uses of 8 Sign-Offs
mid-14c., "serpent's tongue" (thought to be a stinging organ), later "sharp extension of a metal blade" (1680s), from Old Norse tangi "spit of land, pointed metal tool," perhaps related to tunga "tongue" (see tongue). Figurative sense of "a sharp taste" is first recorded mid-15c.; that of "suggestion, trace" is from 1590s. The fish (1734) so called for their spines.