'full-power' is a Modern English compound of the words 'full' and 'power', formed by joining 'full' (meaning 'complete' or 'filled') with 'power' (meaning 'ability, force, or energy').

'full' comes from Old English 'full' meaning 'filled, abundant'; 'power' comes from Old French 'pouvoir'/'poeir' (Middle French 'pouvoir'), ultimately from Latin roots related to 'posse'/'potere' meaning 'to be able'. The compound 'full power' developed in Modern English to express the idea of complete or maximum power and is often hyphenated as 'full-power' when used attributively.

Initially the two words described separate concepts ('full' = 'complete', 'power' = 'ability/force'), but over time the compound came to be used as a single expression meaning 'maximum available power' and as an attributive adjective meaning 'operating at maximum power'.