Crumble
late Middle English: probably from an Old English word related to crumb.
wiktionary
From earlier crymble, crimble, from Middle English *crymblen, kremelen, from Old English *crymlan(“to crumble”), from *crymel(“a small crumb; crumble”), diminutive of Old English cruma(“crumb”), equivalent to crumb + -le(diminutive suffix). Compare Dutch kruimelen(“to crumble”), German Low German krömmeln(“to crumble”), German Krümel, diminutive of German Krume, German krümeln, krümmeln(“to crumble”). Alteration of vowel due to analogy with crumb.
etymonline
crumble (v.)
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"].
