"sluggish, stupefied," with or as with a narcotic drug; also "stupid" generally, 1896, from dope (n.) + -y (2). Related: Dopiness.
Entries linking to dopey
dope n.
1807, American English, "sauce, gravy; any thick liquid," from Dutch doop "thick dipping sauce," from doopen "to dip" (see dip (v.)). Used generally by late 19c. for any mixture or preparation of unknown ingredients.
Extension to "narcotic drug" is by 1889, from practice of smoking semi-liquid opium preparation. Meaning "foolish, stupid person" is older than this (1851) and may be from the notion of "thick-headed," later associated with the idea of "stupefied by narcotics."
Sense of "inside information" (1901) may come from knowing before the race which horse had been drugged to influence performance (to dope (v.) in this sense is attested by 1900). Dope-fiend is attested from 1896, "a victim of the opium habit."
-y 2
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy). Adjectives such as hugy, vasty are artificial words that exist for the sake of poetical metrics.