Green’s Dictionary of Slang

ATM n.2

[ass to mouth]

(US) in pornography, an act of fellatio that immediately follows the use of the same penis in anal sex.

Las Vegas Weekly 8 Jan. 🌐 Porn Glossary [...] ATM -- In the porn industry, this doesn’t mean a bank cash machine. It means taking something that was previously in one person's bottom and then putting that thing directly into the mouth.