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1680s, "pointed stake (for defense against cavalry, etc.)," from French piquet "pointed stake," from piquer "to pierce" (see pike (n.2)). Sense of "troops posted to watch for enemy" first recorded 1761; that of "striking workers stationed to prevent others from entering a factory" is from 1867. Picket line is 1856 in the military sense, 1945 of labor strikes.
1745, "to enclose with pickets," from picket (n.). The sense in labor strikes, protests, etc., is attested from 1867. Related: Picketed; picketing.