Crumb
Old English cruma, of Germanic origin; related to Dutch kruim and German Krume . The final -b was added in the 16th century, perhaps from crumble but also influenced by words such as dumb, where the original final -b is retained although no longer pronounced.
wiktionary
From Middle English crome, cromme, crumme, crume, from Old English cruma(“crumb, fragment”), from Proto-Germanic *krumô, *krūmô(“fragment, crumb”), from Proto-Indo-European *grū-mo-(“something scraped together, lumber, junk; to claw, scratch”), from *ger-(“to turn, bend, twist, wind”). The b is excrescent, as in limb and climb, appearing in the mid 15th century to match crumble and words like dumb, numb, thumb. Cognate with Dutch kruim(“crumb”), Low German Krome, Krume(“crumb”), German Krume(“crumb”), Danish krumme(“crumb”), Swedish dialectal krumma(“crumb”), Swedish in kråm(“crumbs, giblets”), Icelandic krumur(“crumb”), Latin grūmus(“a little heap”).
etymonline
crumb (n.)
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.
