thong n.

1540s, "a pinch, a nipping," from obsolete verb twinge "to pinch, tweak," from Old English twengan "to pinch," from Proto-Germanic *twangjan (source also of Old Frisian thwinga, Old Norse þvinga, Danish tvinge, Dutch dwingen, Old High German thwingan, German zwingen "to compel, force"), from PIE *twengh- "to press in on" (see thong). Meaning "sharp, sudden minor pain" is recorded from c. 1600. Figurative sense (with reference to shame, remorse, etc.) is recorded from 1620s.

"penis," 1933, slang, probably from whangdoodle, an earlier term for "gadget, thing for which the correct name is not known." Many such words (thingy, dingus, etc.) have been used in slang for "penis," not because the actual name was unknown, but because it was unmentionable. Another possibility is that the slang word is a variant of whang "large, thick slice" (1630s), which earlier was used in the sense of "thong" (1530s) and is itself a variant of thwang, an alternative form of thong (see thong). In Old English, wang meant "cheek, jaw," hence wangtoð "cheek-tooth, molar."