A butterfingers is someone with a clumsy tendency to drop things they're holding. Being a butterfingers is considered a particularly bad trait in baseball, for obvious reasons.
The common use of this term by sportscasters in the 1920s inspired the name for the newly-invented candy known as Butterfinger. Before that, many people credited Charles Dickens with coining the word in The Pickwick Papers, in the mouth of a character watching an athlete drop a ball. However, word sleuths have traced butterfingers back at least as far as a 1615 book that described a "good housewife" this way: "she must not be butter-fingered."