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1590s, alteration of Middle English skerren (c.1200), from Old Norse skirra "to frighten; to shrink from, shun; to prevent, avert," related to skjarr "timid, shy, afraid of," of unknown origin. In Scottish also skair, skar, and in dialectal English skeer, skear, which seems to preserve the older pronunciation. To scare up "procure, obtain" is first recorded 1846, American English, from notion of rousing game from cover. Related: Scared; scaring.
"something that frightens; sudden panic, sudden terror inspired by a trifling cause, false alarm," 1520s, alteration of Middle English sker "fear, dread" (c.1400), from scare (v.). Scare tactic attested from 1948.