a dirty rodeo is the act of having sex with your partner and then stopping holding their arms where they cannot escape and say "i have AIDS" or any other type of disease which makes the partner go wild.
when a boy is sitting in jail and is making up words to send in letters back and forth this one is called dirty rodeo where you are fucking a girl in her anus and then grab ahold of her hair gently and whisper in her ear "i have A.I.D.S. and i got it from your sister" and then hurry and grab her hair and pull because she is about to go crazy and want to kill you.