From Greek kolaphos (a slap) through Latin and French — literally a blow, as in a blow struck against the state
A sudden, illegal seizure of government power, or a notably successful action or achievement
From French 'coup' meaning a blow or strike, from Old French 'colp', from Late Latin 'colpus', an altered form of Latin 'colaphus' meaning a blow with the fist, from Greek 'kolaphos' meaning a slap or punch. The phrase 'coup d'etat' (blow to the state) entered English in the 18th century for the overthrow of a government. The shorter form coup is used both for political overthrows and, more casually, for any surprisingly successful move. Key roots: kolaphos (Greek: "a blow with the fist, a slap").
The silent p in coup comes from French, where the word is also pronounced without the final consonant. Spanish and Portuguese transformed the same Latin root into golpe, which looks very different but follows a regular sound change where Latin initial c- became g- in Iberian Romance languages. Spanish golpe de estado is the exact equivalent of coup d'etat.