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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.
(Or tayste /tayst/) Silly suggested term for two binary digits.
The term "quarter" has also been suggested, referring to the US 25-cent coin. This was once equal in value to two of the eight "bits" - pie-slice-shaped "pieces of eight" - into which Spanish silver crowns were cut to make change.