'not-a-bug' originates from Modern English, specifically the words 'not' + 'a' + 'bug', where 'not' meant negation, 'a' is the indefinite article, and 'bug' came to mean a software defect (by extension from earlier senses).

'bug' changed from the Middle English word 'bugge' (originally meaning a frightening creature or specter) to later mean 'insect', then 'fault in a machine' in engineering usage; in computing it became 'software defect'. The phrase 'not a bug' arose in late 20th-century software-development jargon and was later commonly written as the hyphenated compound 'not-a-bug'.

Initially, 'bug' referred to a scary creature (and later an insect); over time it evolved to mean a fault in machinery and then a software defect. 'Not-a-bug' developed to label reported behavior that is judged correct or intended rather than erroneous.