Old English þurst, from Proto-Germanic *thurstu- (source also of Old Saxon thurst, Frisian torst, Dutch dorst, Old High German and German durst), from Proto-Germanic verbal stem *thurs- (source also of Gothic thaursjan, Old English thyrre), from PIE root *ters- "to dry." Figurative sense of "vehement desire" is attested from c. 1200.
Old English þyrstan "to thirst, thirst after," from the noun (see thirst (n.)); the figurative sense of the verb was present in Old English. Compare Old Saxon thurstian, Dutch dorsten, Old High German dursten, German dürsten, all verbs from nouns. Related: Thirsted; thirsting.