c.1200, from O.N. skirra "to frighten," related to skjarr "timid, shy," of unknown origin. The noun is attested from 1520s. To scare up "procure, obtain" is first recorded 1846, Amer.Eng., from notion of rousing game from cover.
p.t. and pp. of scare. Scared stiff first recorded 1900; scared shitless is from 1936. Scaredy-cat "timid person" first attested 1933, in Dorothy Parker.