scary adj.

1520s, "something that frightens, a scarecrow;" 1540s, "sudden panic, sudden terror inspired by a trifling cause, false alarm," from scare (v.). The earlier form was Middle English sker "fear, dread, terror, fright" (c. 1400). Scare tactic "attempt to manipulate public opinion by exploitation of fear" is by 1948.

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.

1845, "timidly," from scary + -ly (2). By 1967 in a positive sense, "unnervingly" (as in scarily good, etc.).