'energy-inefficient' is a modern compound formed from the noun 'energy' and the adjective 'inefficient'. 'energy' originates from Greek, specifically the word 'energeia', via Latin 'energia' and Old/Middle French 'energie'. 'inefficient' derives from the Latin prefix 'in-' (not) combined with 'efficient' (from Latin 'efficere', to bring about/produce) through Old French/Latin into English.

'energy' changed from Greek 'energeia' to Latin 'energia', passed into Middle French as 'energie', and eventually became the modern English word 'energy'. 'inefficient' developed by attaching the negative prefix 'in-' to 'efficient' (from Latin roots such as 'efficere'/'efficax'), producing the English adjective 'inefficient', and the compound 'energy-inefficient' arose in modern English usage.

Initially, 'energy' often referred to general activity or operation and 'inefficient' meant 'not producing the desired effect'; over time the compound 'energy-inefficient' has come to specifically mean 'using more energy than necessary' or 'causing wasteful energy consumption'.