A sex act in which a person (male or futanari) rubs their penis in the cleavage between their partner's breasts. Also known as titty fucking or mammary intercourse. On Bleachbooru the Japanese term 'paizuri' (ぱいずり) is used.
If the penis is long enough, fellatio may also be involved at the same time.
The following tags are aliased to this tag: titfuck and titty_fuck (learn more).