'fatigue-reducing' originates from English, specifically the noun 'fatigue' and the verb 'reduce' (present participle 'reducing'), where 'fatigue' comes from French 'fatigue' meaning 'weariness' and 'reduce' ultimately comes from Latin 'reducere' (with 're-' meaning 'back' and 'ducere' meaning 'to lead').

'fatigue-reducing' formed in modern English by compounding the noun 'fatigue' (from French) with the present participle form of 'reduce' (from Latin via Old French/Middle English); related earlier expressions included phrases like 'reduction of fatigue' and 'fatigue-relieving', which eventually allowed the shorter hyphenated compound.

Initially the component words referred separately to 'weariness' (fatigue) and 'leading back/lessening' (reduce); combined as a compound adjective they came to mean 'causing a decrease of weariness' or 'able to relieve tiredness'.