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.