The definition of a crumb is a tiny bit of something.(noun)
An example of a crumb is a morsel of cake.
Crumb is defined as covering with tiny particles of food.(verb)
An example of crumb is to add a mixture of crushed cookies to the top of an apple pie.
See crumb in Webster's New World College Dictionary
Origin: ME crome < OE cruma, lit., scraping from bread crust, akin to Ger krume < IE *gr-eu, to scratch, scrape (> Ger krauen, to scratch) < base *ger-, to turn, twist
See crumb in American Heritage Dictionary 4
Origin: Middle English crome, from Old English cruma.
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