When a male or futanari ejaculates while being anally (or vaginally, in the case of futanari) penetrated by either an object or a body part.
It should NOT be confused for when a female ejaculates, in which case female ejaculation should be used instead. It also should NOT be confused with penis ejaculation inside a vagina that is being penetrated (cum in pussy). The person ejaculating is the one being penetrated, not otherwise.
The following tags are aliased to this tag: cum_while_penetrated and ejaculation_when_penetrated (learn more).
This tag implicates ejaculation (learn more).