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.
late 14c., "to thicken or contract into a hard covering" (intransitive); see crust (n.). From 1540s in transitive sense of "cover with a crust or hard exterior portion." Related: Crusted; crusting.