'lie-detector' originates from Modern English, formed by compounding the noun 'lie' and the agent noun 'detector' (from 'detect'). 'Lie' originally meant 'false statement,' and 'detector' means 'one that detects.'

'lie' comes from Old English (e.g. 'lēog') meaning 'falsehood'; 'detect' comes from Latin 'detegere' (de- 'off, away' + tegere 'to cover') via Middle French/Latin, with the agent suffix '-or' producing 'detector'. The compound 'lie-detector' developed in Modern English as devices for detecting deception were invented in the 20th century.

Initially the compound straightforwardly meant 'a detector of lies'; over time it became the common name for instruments such as the polygraph and is often used metonymically for the polygraph itself.