also gob-smacked, "flabbergasted, amazed, astounded," literally "smacked in the mouth," by 1936, U.K. slang, from gob (n.2) "mouth" + past participle of smack (v.).
Entries linking to gobsmacked
gob n.2
"mouth," 1540s, from Irish gob "mouth," and thus related to the other English noun gob (also see gobbet). Gob-stopper "type of large hard candy" is from 1928.
smack n.1
"a taste, flavor, savor" especially a slight flavor that suggests something, Middle English smakke, from Old English smæc "taste; scent, odor," from Proto-Germanic *smakka- (source also of Old Frisian smek, Middle Dutch smæck, Dutch smaak, Old High German smac, German Geschmack, Swedish smak, Danish smag), from verb *smakjanan, from a Germanic and Baltic root meaning "to taste" (source also of Lithuanian smaguriai "dainties," smagus "pleasing").
The transferred meaning "a trace (of something)" is attested from 1530s.