Tereus was a son of Ares and a king of the Thracians. He it was who raped Philomela and then cut out her tongue to prevent her revealing what he had done (see her name in connection with this). Perhaps with reference to this incident, or to his nature in general, his name can be derived from teros, 'guard', 'watch'.

Thomas Keightley prefers an origin in thereutes, 'hunter', from thereuo, 'to hunt', 'run down'. (But could this be a reference to his chase after Philomela and Procne, and their consequent metamorphosis into a nightingale and a swallow? See the sisters' names for more about this.)