I did the search 1st & couldn't believe you guys haven't posted any "Dirty Johnny" jokes. (there have to be millions!!!) - here we'll see if can jog your memories: (also known as little Johnny)
A teacher is polling her 2nd grade class asking the students to use a word in a sentence with a letter from the alphabet - in order A-Z. On each letter dirty Johnny is raising his had, but she doesn't dare pick him because everything he associates with is rude.
When she gets to "R" no-one but Johnny has his hand up, so with reluctance (and her brain racing to think of a vulgarity with "R") she picks him.
He stands up and says "I saw a rat... R A T"
She says "very good Johnny" (with a great sigh of relief)
He adds " a big mother fucking rat, with a dick this big"