To fall apart; to disintegrate.
To break into crumbs.
To mix (ingredients such as flour and butter) in such a way as to form crumbs.
A dessert of British origin containing stewed fruit topped with a crumbly mixture of fat, flour, and sugar.
To weaken someone's morale over a long period.
To make something hard smaller and smoother by gradually rubbing off tiny bits, to wear down.