Ass to mouth is a common colloquial term for the removal of the penis from the passive partner's ass followed by the immediate insertion of the penis into the passive partner's or another partner's mouth.
It is also known as A2M, ATM, ATG, Ass-to-Mouth or Arse-to-Gob (Scottish colloquial). Ass-to-mouth generally excludes cleaning the penis or other object after its removal from the anus and before its insertion into the mouth. This may enhance sexual excitement for both active and passive partners, often developing from connotations of domination/humiliation. Ass-to-Other-Mouth or Ass-to-Other-Girl-Mouth, abbreviated as A2OM, ATOM, A2OGM or ATOGM, specifically describes the variant of the act where the penis is moved from one partner's anus to a different partner's mouth. Another variation is known as A2P, ATP, or Ass-to-Pussy which refers to removing the penis from the anus and inserting it into the vagina. The term has been used since at least January 1995 and clearly defined since at least August 1996.
In the film Clerks II the main characters have a discussion on ass to mouth early in the film, and it is alluded to throughout the film. The main character, Dante, repeatedly warns Randal, "You never go ass to mouth!"