early 15c., "fragment, piece torn off," also "strip of cloth," a northern England dialectal variant of Old English screade (see shred (n.)). Meaning "lengthy speech" is by 1812, from the notion of reading from a long list or simply a "long strip" of speaking.
Middle English crome, crumme, from Old English cruma "fragment of bread or other food, a morsel, small fragment," from a West Germanic root of obscure origin (compare Middle Dutch crume, Dutch kruim, German Krume); perhaps from a PIE word for "small particle of bread" and cognate with Greek grumea "bag or chest for old clothes" (Beekes writes: "In origin, the word probably denoted small things of little value, later also the chest, etc.), Albanian grime.
The unetymological -b- appeared mid-15c., in part by analogy with words like dumb. Slang meaning "lousy person" is 1918, from crumb, U.S. slang for "body-louse" (1863), which were so called from resemblance.
late 13c., "a bite, mouthful; small piece of food, fragment," from Old French morsel (Modern French morceau) "small bite, portion, helping," diminutive of mors "a bite," from Latin morsum, neuter of morsus "biting, a bite," past participle of mordēre "to bite," which is perhaps from an extended form of PIE root *mer- "to rub away, harm."