protest (n.)
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).