Something that's crisp is thin, dry, and breakable. Unless they're stale, most crackers are crisp.
Gingersnaps are crisp, and so is toast — in fact, crisp often describes food, although crunchy snow and dry, brown leaves are also crisp. To crisp something is to cook or dry it until it's brittle, and when a voice is crisp, it's abrupt and no-nonsense. A crisp fall morning is fresh and cold. In Old English, crisp meant "curly," from the Latin crispus, "curled." The "brittle" definition is probably imitative, the word sounding like its meaning.