crus·ta·ceous \|krə|stāshəs\adjective Etymology: New Latin crustaceus, from Latin crusta shell, crust + -aceus -aceous — more at crust 1.: of the nature of, having, or suggesting a crust or shell 2.: crabby II 1, crusty 1a(2) 3.: belonging to the Crustacea : crustacean 4.botany a.: having a brittle crust b.of a lichen: having a crusty thallus adhering inseparably to rocks, bark, soil