early 14c., "hard outer part of bread," from Old French crouste (13c., Modern French croûte) and directly from Latin crusta "rind, crust, shell, bark," from PIE *krus-to- "that which has been hardened," suffixed form of root *kreus- "to begin to freeze, form a crust." From mid-15c. as "any hard, external portion of comparative thinness;" meaning "outer shell of the earth" is from 1550s.
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).
<a href="https://www.etymonline.com/word/crusty">Etymology of crusty by etymonline</a>
Harper, D. (n.d.). Etymology of crusty. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/crusty