When somebody is taking a nap outdoors naked, anus exposed in a relaxed (slightly open) state then a fly comes along and lands in their anus, causing it to reflexively close. Trapping the fly inside.
"I couldn't figure out why my butt itched so much after camping yesterday , after a bit of finger digging I found out that I am part anus fly trap!"
The act of sealing your lips around someone else's anus and sucking out gasses and debris without the anus inhalee trying push out the poo. Sometimes involves semen, in which case it is referred to as a wet vac.
"Oh my, Floyd just inhaled from the anus of Jay without even coughing, he's a damn good experienced anus inhaler. We shall see how Jay does tomorrow, he is much more experienced, probably has fully coated his lungs in poo by now, I bet he doesn't even taste the farts anymore, he's goin straight for the corn layer next."