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.
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).