Of poor quality, disagreeable, or unpleasant; not good at all.
Possibly from 'crumb' meaning something small and worthless, or from slang 'crumb' meaning a disagreeable person. The exact origin is uncertain but the sense of worthlessness is clear.
A 'crummy' day is so common in casual speech that few people remember it might come from actual crumbs—small, insignificant, and not worth much.
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