the.com/protestor

means A person who publicly objects to something — often by marching, chanting, or gathering — in hopes of changing minds or policy.

from Built from 'protest,' which traces back to the Latin protestari — 'pro' (forth) plus 'testari' (to testify, bear witness, related to 'testimony'). To protest originally meant to formally declare or affirm something publicly, like testifying in front of others; only later did it narrow to mean objecting or opposing. So a protestor is, etymologically, someone bearing public witness — the sign-waving is just the modern form of testifying.