'energy-wasteful' is a modern English compound formed from the words 'energy' and 'wasteful'. 'energy' originates from Greek, specifically the word 'energeia', where 'energeia' meant 'activity, operation'. 'wasteful' comes from 'waste' + the suffix '-ful'; 'waste' ultimately comes from Old North French/Old French roots (compare Old French 'waste'/'gast') meaning 'desolate, to squander', and '-ful' is an Old English/Old Germanic suffix meaning 'full of'.

'energy' changed from Greek 'energeia' into Late Latin/Medieval Latin forms (e.g. 'energia'), then into Middle English 'energie' and eventually the modern English 'energy'. 'wasteful' developed from Old English/Old French roots for 'waste' through Middle English into the adjective formed with the native English suffix '-ful', producing 'wasteful' in modern English; the compound 'energy-wasteful' is a recent, transparent descriptive compound combining those two elements.

Individually, 'energeia' originally meant 'activity, operation', and 'waste' originally meant 'desolate' or 'to squander'; combined as 'energy-wasteful' the modern meaning became 'using or causing unnecessary consumption of energy.'