1. hard outer coating of a loaf of bread; piece of bread which is mainly crust or that is hard and dry; hard coating, hard outer layer; (Geology) outer portion of the earth. cover with a crust, form into a crust; be formed into a crust. crustcrust (kr?st), n. [l. crusta: cf. of. crouste, f. croûte; prob. akin to gr. ice, e. crystal, from the same root as e. crude, raw. see: raw, and cf. custard.]