'polygraph' originates from Greek, specifically the roots 'poly-' and 'graphein', where 'poly-' meant 'many' and 'graphein' meant 'to write'.

'polygraph' was coined in modern English from Greek roots in the 19th century; it was used both for devices that make multiple recorded traces and, archaically, for a person who writes on many subjects. In the 20th century the term came to be associated especially with instruments for recording physiological signals used in lie-detection.

Initially it conveyed the idea of 'many-writing' (a device/person producing multiple written records); over time it shifted to mean specifically a machine that records multiple physiological signals and, by extension, the lie-detector test performed with that machine.