Since this is not a complete sentence, we must choose a noun to change to butt, preferably one near the end. The obvious choice here is "sentence." Then we would have:
Using the word "butt" in my butt
This version loses a lot of the original meaning, however, especially since the word "butt" is already used previously in the sentence. Another thing to try might be:
Using the word "butt" in any of my butts
Sometimes it is necessary to add a word or two for increased comic value.
This sentence has no predicate noun. In that case, we can make the predicate adjective describe my butt instead:
My friend Bob is my really neat butt.
We can also try:
My friend Bob is really my neat butt.
In this case we might want to try modifying the object of the preposition:
Massachusetts Institute of My Butt
Mr. Sulu, set a course for my butt.
New examples are welcome--send suggestions to annied@mit.edu.