'bug-free' is a modern compound formed from the noun 'bug' + the adjective 'free'. 'Bug' in this sense (insect or technical defect) became common in English by the 19th and 20th centuries, and 'free' originates from Old English 'freo' meaning 'not subject to, exempt'.

The element 'bug' derives from Middle English 'bugge' (a frightening being, hobgoblin) and later came to mean 'insect' and then 'mechanical or technical defect' (19th–20th century); 'free' comes from Old English 'freo' and Germanic roots. The compound 'bug-free' arose in the 20th century as concerns about pests and later software defects became common.

Initially compounds with 'free' typically described absence of physical things (e.g. 'disease-free', 'pest-free'); over time 'bug-free' expanded from meaning 'without insects' to include 'without technical defects' (especially in computing), and now commonly refers to software releases with no known bugs.