1.  The hard external coat or covering of anything; the hard exterior surface or outer shell; an incrustation; as, a crust of snow. [1913 Webster]
"I have known the statute of an emperor quite hid under a crust of dross." [1913 Webster]
"Below this icy crust of conformity, the waters of infidelity lay dark and deep as ever." [1913 Webster]

2.  The hard exterior or surface of bread, in distinction from the soft part or crumb; or a piece of bread grown dry or hard. [1913 Webster]
"Th' impenetrable crust thy teeth defies." [1913 Webster]
"He that keeps nor crust nor crumb." [1913 Webster]
"They . . . made the crust for the venison pasty." [1913 Webster]

3.  The exterior portion of the earth, formerly universally supposed to inclose a molten interior. [1913 Webster]

4.  The shell of crabs, lobsters, etc. [1913 Webster]

5.  A hard mass, made up of dried secretions blood, or pus, occurring upon the surface of the body. [1913 Webster]

6.  An incrustation on the interior of wine bottles, the result of the ripening of the wine; a deposit of tartar, etc. See Beeswing. [1913 Webster]

   To cover with a crust; to cover or line with an incrustation; to incrust. [1913 Webster]
"The whole body is crusted over with ice." [1913 Webster]
"And now their legs, and breast, and bodies stood
Crusted with bark." [1913 Webster]
"Very foul and crusted bottles." [1913 Webster]
"Their minds are crusted over, like diamonds in the rock." [1913 Webster]

   To gather or contract into a hard crust; to become incrusted. [1913 Webster]
"The place that was burnt . . . crusted and healed." [1913 Webster]

crust, n. & v.
--n.
1 a the hard outer part of a loaf of bread. b a piece of this with some soft bread attached. c a hard dry scrap of bread. d esp. Austral. sl. a livelihood (what do you do for a crust?).
2 the pastry covering of a pie.
3 a hard casing of a softer thing, e.g. a harder layer over soft snow.
4 Geol. the outer portion of the earth.
5 a a coating or deposit on the surface of anything. b a hard dry formation on the skin, a scab.
6 a deposit of tartar formed in bottles of old wine.
7 a sl. impudence (you have a crust!). b a superficial hardness of manner.
--v.tr. & intr.
1 cover or become covered with a crust.
2 form into a crust.

Derivative:
crustal adj. (in sense 4 of n.).

Etymology:
ME f. OF crouste f. L crusta rind, shell

N  covering, cover, baldachin, baldachino, baldaquin, canopy, tilt, awning, tent, marquee, tente d'abri, umbrella, parasol, sunshade, veil (shade), shield, roof, ceiling, thatch, tile, pantile, pentile, tiling, slates, slating, leads, barrack, plafond, planchment, tiling, shed, top, lid, covercle, door, operculum, bulkhead, bandage, plaster, lint, wrapping, dossil, finger stall, coverlet, counterpane, sheet, quilt, tarpaulin, blanket, rug, drugget, housing, antimacassar, eiderdown, numdah, pillowcase, pillowslip, linoleum, saddle cloth, blanket cloth, tidy, tilpah, apishamore, integument, tegument, skin, pellicle, fleece, fell, fur, leather, shagreen, hide, pelt, peltry, cordwain, derm, robe, buffalo robe, cuticle, scarfskin, epidermis, clothing, mask, peel, crust, bark, rind, cortex, husk, shell, coat, eggshell, glume, capsule, sheath, sheathing, pod, cod, casing, case, theca, elytron, elytrum, involucrum, wrapping, wrapper, envelope, vesicle, corn husk, corn shuck, dermatology, conchology, testaceology, inunction, incrustation, superimposition, superposition, obduction, scale, veneer, facing, overlay, plate, silver plate, gold plate, copper plate, engobe, ormolu, Sheffield plate, pavement, coating, paint, varnish, plating, barrel plating, anointing, enamel, epitaxial deposition, vapor deposition, ground, whitewash, plaster, spackel, stucco, compo, cerement, ointment, covering, superimposed, overlaid, plated, cutaneous, dermal, cortical, cuticular, tegumentary, skinny, scaly, squamous, covered, imbricated, loricated, armor plated, ironclad, under cover, cowled, cucullate, dermatoid, encuirassed, hooded, squamiferous, tectiform, vaginate.

earth's crust, pie crust, upper crust