"one who wins a country, subjugates a people, or defeats an adversary," c. 1300, from Anglo-French conquerour, Old French conquereor, from Old French conquerre "conquer, defeat, vanquish," from Vulgar Latin *conquaerere (for Latin conquirere) "to search for, procure by effort, win," from assimilated form of Latin com-, here perhaps an intensive prefix (see com-), + quaerere "to seek, gain" (see query (v.)).
Another early form was conquestor, from the Latin agent noun, conquistor, conquaestor. Fem. form conqueress is attested from c. 1400. William Duke of Normandy was called William the Conqueror from early 12c. in Anglo-Latin (Guillelmus Magus id est conqustor rex Anglorum), by late 14c. in English.
-er 1
English agent noun ending, corresponding to Latin -or. In native words it represents Old English -ere (Old Northumbrian also -are) "man who has to do with," from Proto-Germanic *-ari (cognates: German -er, Swedish -are, Danish -ere), from Proto-Germanic *-arjoz. Some believe this root is identical with, and perhaps a borrowing of, Latin -arius (see -ary).
Generally used with native Germanic words. In words of Latin origin, verbs derived from past participle stems of Latin ones (including most verbs in -ate) usually take the Latin ending -or, as do Latin verbs that passed through French (such as governor); but there are many exceptions (eraser, laborer, promoter, deserter; sailor, bachelor), some of which were conformed from Latin to English in late Middle English.
The use of -or and -ee in legal language (such as lessor/lessee) to distinguish actors and recipients of action has given the -or ending a tinge of professionalism, and this makes it useful in doubling words that have a professional and a non-professional sense (such as advisor/adviser, conductor/conducter, incubator/incubater, elevator/elevater).