crumb n.

late 15c., kremelen, "to break into small fragments" (transitive), from Old English *crymelan, presumed frequentative of gecrymman "to break into crumbs," from cruma (see crumb). Intransitive sense of "fall into small pieces" is from 1570s.

The -b- is from 16c., probably on analogy of crumb (where it also is an unetymological intrusion) or of French-derived words like humble, where it belongs. Related: Crumbled; crumbling. Old English gecrymman yielded Middle English crimen "to crumble" (transitive).

As a noun, from 1570s as "a fragment," from 1947 in cookery as dessert dish with a crumb topping, British English. "The technique itself seems to have been a product of Second World War culinary making-do" [Ayto, "Diner's Dictionary"].

"full of crumbs," 1731, from crumb + -y (2). Overlapping somewhat with crummy, but generally restricted to the more literal senses.

1560s, "easily crumbled;" 1570s, "like bread," from crumb + -y (2). Slang meaning "shoddy, filthy, inferior, poorly made" was in use by 1859, probably from the first sense, but influenced by crumb in its slang sense of "louse." The "like bread" sense probably accounts for 18c. (and later in dialects) use, of a woman, "attractively plump, full-figured, buxom."  Related: Crummily; crumminess.