Adynaton

Adynaton (plural adynata) is a figure of speech in the form of hyperbole taken to such extreme lengths as to insinuate a complete impossibility:

The word derives from the Greek ἀδύνατον (adunaton), neuter of ἀδύνατος (adunatos), "unable, impossible" (a-, "without" + dynasthai, "to be powerful").