Crusty (-), a.
1. Having the nature of crust; pertaining to a hard covering; as, a crusty coat; a crusty surface or substance.
2. Etym: [Possibly a corruption of cursty. Cf. Curst, Curstness.]
Having a hard exterior, or a short, rough manner, though kind at heart; snappish; peevish; surly. Thou crusty batch of nature, what's the news Shak.
Shades of meaning
Fernald has no entry of its own for crusty; he lists it under morose.
He lists it without comment; the entries above explain its neighbors.