It is generally agreed that the recently deceased Bo Diddley was not born with that name. Where he got it is a subject of conjecture.
An Associated Press obituary of the pioneer rock-and-roller says that he was born in Mississippi as Elias Bates, and: “The name Bo Diddley came from other youngsters when he was growing up in Chicago, he said in a 1999 interview. However, over time he gave somewhat differing stories on where he got the name. Some experts believe a possible source for the name is a one-string instrument used in traditional blues music called a diddley bow.”
Wikipedia mentions the diddley bow theory, but concludes that Bo Diddley “is probably a southern black phrase meaning ‘nothing at all,' as in ‘He ain't bo diddley.' ”
Wikipedia is on the right track, I think. I'm reasonably sure that I'd heard bo diddley before I heard of the performer who called himself such, and I know I'd heard diddley and diddley squat, both of which were commonly used to mean “nothing” or “worthless” or “excrement,” as in “He don't know diddley [squat].” About the same time Bo Diddley was becoming famous, another early rock-and-roller was singing “My girl is red hot, your girl ain't diddley squat.” Or maybe it was “doodley squat.” Same thing.
While looking up diddley squat in the Dictionary of American Regional English, I came across diddy-wah-diddy, which happens to be the name of another Bo Diddley song. DARE says that diddy-wah-diddy is a Southernism used as “the name of an imaginary place, often conceived of as fabulous and far-off.”
“Oh, how they love in Diddy-Wah-Diddy,” Bo Diddley sang. A little earlier, another Southern artiste, Phil Harris, employed a variation: “Let me tell you 'bout a place called Doo-Wah-Diddy/It ain't a town and it ain't a city/But it's awful nice and it's awful pretty/and that's what I like about the South.”