crust Look up crust at Dictionary.com
early 14c., from L. crusta "rind, crust, shell, bark," from PIE *krus-to-, from base *kreus- "to begin to freeze, form a crust" (cf. Skt. krud- "make hard, thicken;" Avestan xruzdra- "hard;" Gk. krystallos "ice, crystal," kryos "icy cold, frost;" Lett. kruwesis "frozen mud;" O.H.G. hrosa "ice, crust;" O.E. hruse "earth;" O.N. hroğr "scurf"). Meaning "outer shell of the earth" is from 1550s.