A phrase used when one's significant other is receiving anal pleasure, preferably in a public place, such as a driveway, from someone's cock that is not one's own. Normally, a large crowd of onlookers has gathered. These circumstances cause one to, when describing the circumstances to another, misspeak the phrase "cock in her ass," with "ass in her cock."
William H. Macy's character in "Boogie Nights," reacting to his wife's infidelity: "My fucking wife has an ass in her cock over in the driveway, alright? I'm sorry if my thoughts aren't with the photography of the film we're shooting tomorrow, Kurt, OK?"