"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 *khnudojanan. 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.
mid-15c., from nod (v.). Land of Nod "sleep" is a pun on the biblical place name (Gen. iv:16).
To be intoxicated with narcotics to a very drowsy or stuporous state: with slews of rich kids nodding in the Scarsdale woods
[1960s+ Narcotics; the underlying sense, ''let the head fall forward when drowsy,'' is found by 1562]