manager

Manager comes from manage plus the agent suffix -er. Manage traces back through Old French manege and Old Italian maneggiare to Vulgar Latin manizare, itself built on Latin manus, meaning hand.

The word's root sense was physical handling — literally working something with the hand — and this survived into manage, the verb for handling or directing affairs. Adding -er turned that action into a person: one who does the handling.

The -er ending has a long pedigree of its own, descending through Old English -ere and Proto-Germanic *-ārijaz, the same suffix that turns countless English verbs into 'doer' nouns.