Emily Dickinson Lexicon

CRUST, n. [L. crusta; Fr. croûte; It. crosta; D. korst; G. kruste; W. crest, from cresu, to parch or scorch, cres, a hardening by heat. But the primary sense is probably to shrink, contract, harden, whether by cold or heat, and it is probably allied to crystal, freeze, crisp, &c. See Class Rd, No. 19, 33, 73, 76, 83, 85, 88.]

1. An external coat or covering of a thing, which is hard or harder than the internal substance; as, the crust of bread; the crust of snow; the crust of dross; the crust of a pie.
2. A piece of crust; a waste piece of bread. – Dryden. L'Estrange.
3. A shell, as the hard covering of a crab and some other animals.
4. A scab.
5. The superficial substances of the earth are, in geology, called its crust.