Know how to use "fewer" and "less"? Find out.
Old English cruma "crumb, fragment," from a West Germanic root of obscure origin (cf. Middle Dutch crume, Dutch kruim, German krume). The -b- appeared mid-15c., in part by analogy with words like dumb, in part from crumble. Slang meaning "lousy person" is 1918, from crumb, U.S. slang for "body-louse" (1863), so called from resemblance.