Mid 17th century (in an earlier sense). From post-classical Latin adynaton impossibility, (in rhetoric) figure of speech by which an impossible situation is used for emphasis and its etymon ancient Greek ἀδύνατον impossibility, use as noun of neuter of ἀδύνατος impossible from ἀ- + δυνατός possible from δύνασθαι to be powerful + -τός, suffix forming verbal adjectives.