Etymology
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crust (n.)

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.

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crust (v.)

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.

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crustation (n.)

"an incrustation; an act of crusting over," 1650s, noun of action or state from crust (v.).

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crusty (adj.)

c. 1400, "scabby, like a crust, hard," from crust (n.) + -y (2). Figurative use, of persons, "short-tempered, surly, peevish" is from 1560s.

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*kreus- 
Proto-Indo-European root meaning "to begin to freeze, form a crust."

It forms all or part of: crouton; crust; Crustacea; crustacean; cryo-; cryogenic; crystal; crystalline; crystallize; custard; encrust; Kristallnacht.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit krud- "make hard, thicken;" Avestan xruzdra- "hard;" Greek krystallos "ice, crystal," kryos "icy cold, frost;" Latin crusta "rind, crust, shell, bark;" Lettish kruwesis "frozen mud;" Old High German hrosa "ice, crust;" Old English hruse "earth;" Old Norse hroðr "scurf."
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Crustacea (n.)

arthropod class, 1814, Modern Latin neuter plural of crustaceus (animalia), literally "having a crust or shell," from Latin crusta "crust, rind, bark, hard shell" (from PIE root *kreus- "to begin to freeze, form a crust;" also see -a (2)). Taken as a zoological classification by Lamarck, 1801; Cuvier (1798) had les insectes crustacées.

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crustacean (n.)

an animal of the class Crustacea, 1835; see Crustacea + -an. As an adjective, "of or pertaining to an animal of the class Crustacea," 1858 (an earlier adjective was crustaceous, "pertaining to crust, crust-like," 1640s).

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gratin (n.)
light crust over a dish, 1806 (in au gratin), from French gratin "crust" (16c.), from gratter "to scrape, scratch" (see grate (v.)).
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crouton (n.)

1806, from French croûton "small piece of toasted bread," used in soups, salads, etc., from croûte "crust," from Old French crouste (13c.), from Latin crusta "rind, crust, shell, bark" (from PIE root *kreus- "to begin to freeze, form a crust").

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rind (n.)

Old English rinde "bark of a tree or other plant," also figurative; also "a crust, firm outer coating or covering;" later "peel of a fruit or vegetable" (late 14c.), from Proto-Germanic *rind- (source also of Old Saxon rinda, Middle Dutch and Dutch rinde "bark of a tree," Old High German rinda "crust, bark," German Rinde "crust, crust of bread"), which is perhaps related to Old English rendan (source of rend (v.)); Boutkan suggests the group might be from a PIE root *(H)rendh-. The meaning "skin of a person or animal" (as in pork rind) is by 1510s.

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