'warner' originates from English, formed by the verb 'warn' plus the agentive suffix '-er' (meaning 'one who does X').

'warn' comes from Old English 'warnian' (to give notice, caution), from Proto-Germanic roots; the agent noun 'warner' developed in English by adding the suffix '-er' to the verb.

Initially it meant 'one who gives notice or caution', and this basic meaning has remained largely unchanged in modern usage.