'protest' originates from Latin, specifically the word 'protestari', where 'pro-' meant 'forth/publicly' and 'testari' meant 'to testify or bear witness'.

'protestari' passed into Old French as 'proster'/'protester' and into Middle English as 'protesten', which eventually became the modern English word 'protest'.

Initially it meant 'to declare publicly or testify'; over time it evolved to mean 'to express objection or dissent', and also came to refer to public demonstrations of that objection.