"No Diddy" is a replacmentfor the term "No Homo", a
meme term appended to the end of a phrase which may otherwise be interpretted as
homosexual, to reaffirm your sexuality as straight.
It originated following the court case of rapper Sean John Combs aka Diddy, also known as Puff Daddy and P. Diddy. The case detailed allegations of drug mules and
sex-trafficking; this along with the years of rumour and speculation as to Diddy's sexual orientation,
sex parties and homosexual activities led to the
meme.
No Diddy is also occasionally used outside of this context as an exclamation or for emphasis, similar to "no way", or without any context at all for comedic value one might just exclaim "no diddy?", "no diddy!".