Crusty is from 'crust' + the adjectival suffix '-y'; 'crust' comes via Old French from Latin 'crusta' ('rind, shell, hard surface'). The 'irritable' sense is a figurative hardening. The immediate etymological form is Latin crusta. The suffix -y forms an adjective meaning characterized by the base, so the English form is shaped by recognizable word formation as well as by its inherited or borrowed base. In present use as an adjective, it means "having a hard outer crust; or curt and irritable in manner", keeping the historical formation close to the word's ordinary semantic work. The figurative development named in the history lets the word move from a concrete base into a broader descriptive use. Related words include crust; derived forms include crustily and crustiness.