nipple penetration
When an object is inserted inside one's nipples, thus the nipple being penetrated.
Can include fingers, needles, penises, tentacles and even other nipples.
Many examples can be found in the taimanin asagi series, especially in the taimanin asagi 2 eroge and hentai ova episode 1.
The following tags are aliased to this tag: nipple_fuck (learn more).