"the melting of ice or snow," also "spell of weather causing this," c.1400, from thaw, v.. Figurative sense is from 1590s; specifically "relaxation of political harshness or hostility" from 1950, an image from the "Cold War."

Old English þawian (transitive), from Proto-Germanic *thawon- (cognates: Old Norse þeyja, Middle Low German doien, Dutch dooien, Old High German douwen, German tauen "to thaw"), from PIE root *ta- "to melt, dissolve" (cognates: Sanskrit toyam "water," Ossetic thayun "to thaw," Welsh tawadd "molten," Doric Greek takein "to melt, waste, be consumed," Old Irish tam "pestilence," Latin tabes "a melting, wasting away, putrefaction," Old Church Slavonic tajati "to melt"). Intransitive sense from early 14c. Related: Thawed; thawing.