Definitely

Used to emphasize that something is certain to happen or is unquestionably true; in a definite manner.

Formed from 'definite' plus the adverbial suffix '-ly', first appearing in English around 1574. The root comes from Latin 'definitus', past participle of 'definire' meaning 'to limit, determine, explain', which itself derives from 'de-' (completely) and 'finire' (to bound, limit).