"to quickly bow the head," late 14c., of unknown origin, probably an Old English word, but not recorded; perhaps related to Old High German hnoton "to shake," from Proto-Germanic *hnudan. Meaning "to drift in and out of consciousness while on drugs" is attested from 1968. Related: Nodded; nodding. A nodding acquaintance (1711) is one you know just well enough to greet with a nod.[nod etymology, nod origin, 英语词源]