'out-of-spec' originates from Modern English, specifically the phrase 'out of spec', where 'out' meant 'outside/not within' and 'spec' is an abbreviation of 'specification' (a detailed description of requirements).

'out-of-spec' changed from the fuller technical expression 'out of specification' or 'out of specifications' used in engineering and manufacturing, and over time the shorter hyphenated form 'out-of-spec' became common in industry writing.

Initially, it meant 'outside the stated specification(s)'; over time it has been used more broadly to mean 'not meeting required standards or tolerances' in technical and general contexts.