No, it's not what it sounds like.

This tag is for scenarios in which a character engulfs another character (usually entirely but only strictly requiring the penis) and uses their genitals as their own. This is generally illustrated by the engulfed character's penis physically residing within the larger host's penis. The name is derived from the latter character being reduced to a cum-producer for the former; a living testicle.

Usually involves a goo_creature and themes of body_control, mind_control or merging. Although the name may imply it, this tag does not require visible ejaculation to be applied.