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.
c. 1400, "avowal, pledge, solemn declaration," from Old French protest, from protester, from Latin protestari "declare publicly, testify, protest," from pro- "forth, before" (from PIE root *per- (1) "forward," hence "in front of, before") + testari "testify," from testis "witness" (see testament).
Meaning "statement of disapproval" is recorded by 1751. By late 19c. this was mostly restricted to "a solemn or formal declaration against some act or course of action."
The adjectival sense of "expressing of dissent from, or rejection of, prevailing social, political, or cultural mores" is by 1942, in reference to U.S. civil rights movement (in protest march); protest rally from 1960. Protest vote, "vote cast to demonstrate dissatisfaction with the choice of candidates or the current system," is by 1905 (in reference to Socialist Party candidates).
Because they now fully understood the power of the picket line, they were ready and anxious to march on Washington when A. Philip Randolph, president of the Brotherhood of Sleeping Car Porters, advanced the idea in January 1941 of organizing a Negro protest march on Washington, because Government officials from the President down to minor bureau chiefs, had persistently evaded the issue of combating discrimination in defense industries as well as the Government itself. As the time for the event drew nearer some of the heads of the Government became alarmed; Randolph reported that a ranking New Dealer had told him many Government officials were asking, "What will they think in Berlin?" [Statement of Edgar G. Brown, Revenue Revision of 1942 hearings, 77th Congress, 2nd session]