1680s, "pointed post or stake (usually of wood, for defense against cavalry, etc.)," from French piquet "pointed stake," from piquer "to pierce" (see pike (n.1)). Also "one of a number of pointed bars used to make a fence," hence picket-fence (1817). The sense of "troops posted in front of an army to give notice of the approach of the enemy" is recorded from 1761; that of "striking workers stationed to prevent others from entering a factory" is from 1867. Picket-line is by 1856 in the military sense, by 1945 of labor strikes.
1745, "to enclose or fortify with pointed stakes," from picket (n.). Meaning "to place or post as a guard of observation" is by 1775. The sense in labor strikes, protests, etc., is attested from 1867. Related: Picketed; picketing.