Adynaton

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

I will sooner have a beard grow in the palm of my hand than he shall get one on his cheek.

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