'diminished-power' is a Modern English compound formed from 'diminished' (the past participle of 'diminish') + 'power'. 'diminish' ultimately comes from Latin 'diminuere' (via Old French 'deminuer'/'diminuer'), where 'di-' (variant of 'dis-') and 'minuere' meant 'to make small'. 'power' comes from Old French 'pouvoir' (noun/verb) derived from Latin 'posse' meaning 'to be able.'

'diminish' changed from Latin 'diminuere' to Old French 'diminuer' and Middle English 'diminish(en)', becoming the modern English 'diminish'. 'power' evolved from Latin usage through Old French 'pouvoir' to Middle English 'power'. The compound expression 'diminished power' arose in Modern English as a descriptive phrase and is sometimes hyphenated (e.g., 'diminished-power') for adjectival use.

Individually, 'diminish' originally meant 'to make smaller' and 'power' meant 'ability' or 'authority'; combined as 'diminished power' the phrase came to mean the contemporary sense of 'reduced strength, effectiveness, or authority.'