Crummy means shabby, miserable, or of little value. The word was originally spelled crumby, but crumby is shedding that definition and is increasingly confined to its older senses—(1) full of or covered in crumbs, and (2) tending to break into crumbs. In the second sense, it’s synonymous with crumbly.
Crumby has been in English since the 17th century, and it gained the slang senses now associated with crummy in the 19th century.1 Crummy has several little-used old definitions,2 but it emerged as a newer spelling of crumby in its slang senses only in the second half of the 20th century.3 Crumby still appears in place of crummy much more often than the reverse, but the differentiation is fairly well established this century and is in evidence in newswriting and books from around the English-speaking world. A few examples are below.
Still, the words will probably remain variants of each other because they are so closely related—both come from crumb 4—and because neither is very common, meaning people don’t encounter them enough to feel certain of how they’re spelled.
Even crummy Matt—who was so crummy he threw other kids’ toys into the road—wanted to play with Bean. [Ivy and Bean Book 1, Annie Barrows]
This helps create steam, which lifts and puffs the crumby crust away from the meat, helping it turn golden and intensifying the crunch. [New York Times]
She said asylum seekers would far rather do a day’s work than sit in some crummy little flat or house being paid a pittance and having nothing to do. [The Australian]
[H]e would bring a tray of delicious things up to bed, handing her coffee as she awoke, when they would exchange joyous, crumby kisses. [The Last Letter from Your Lover, Jojo Moyes]
Unhappily, that earlier, fairly crummy picture now seems like FW Murnau’s Sunrise when set beside the blitzkrieg of awfulness that is Nativity 2: Danger in the Manger!. [Irish Times]
I sipped the hot tea while Spider and his nan both sat dunking their biscuits and slurping down soggy, crumby mouthfuls. [Numbers, Rachel Ward]
1. Crumby in the OED ↩
2. Crummy in the OED ↩
3. Google ngram graphing use of the phrase “crummy day” in English-language publications, 1800-2000 (“day” being a noun we chose arbitrarily because it has nothing to do with crumbs) ↩
4. Chambers Dictionary of Etymology ↩