What is the X in X-mas?
Old English crisp "curly," from Latin crispus "curled, wrinkled, having curly hair," from PIE root *(s)ker- "to turn, bend." It began to mean "brittle" 1520s, for obscure reasons, perhaps based on what happens to flat things when they are cooked. Figurative sense of "neat, brisk" is from 1814; perhaps a separate word. As a noun, from late 14c. Potato crisps (the British version of U.S. potato chips) is from 1929.
late 14c., "to curl," from crisp (adj.). Meaning "to become brittle" is from 1805. Related: Crisped; crisping.