noun

1. 1. the hard, crisp outer part of bread
   2. a piece of this
   3. any dry, hard piece of bread
2. the pastry shell of a pie
3. any hard surface layer, as of snow or soil
4. a hard deposit formed by wine on the inside surface of a bottle
5. Slang audacity; insolence; gall
6. Geol. the solid, rocky outer portion or shell of the earth; lithosphere
7. Med. a dry, hard outer layer of blood, pus, or other bodily secretion

Etymology: ME cruste < OFr or L: OFr crouste < L crusta: for IE base see crude

transitive verb, intransitive verb

1. to cover or become covered with a crust
2. to form or harden into a crust