crayfish [ME] A crayfish is not a fish but a freshwater crustacean that looks like a small lobster. Its name came into medieval English from Old French crevice (modern French écrevisse), and was probably related to *crab (although not to crevice [ME], which is from Old French crever, ‘to burst or split’). The spelling was altered in the 16th century simply because people thought that it made more sense: it lives in water, so it must be a fish. Crawfish is a variant that has been around since at least the 17th century, but is now mainly an American usage.