Mammary intercourse is a sex act, performed as either foreplay or as non-penetrative sex, that involves the stimulation of the male penis by the female breasts and vice versa.
Mammary intercourse Wikipedia
Mammary intercourse involves the male placing his erect penis in the female's cleavage, then start rubbing or thrusting between her breasts, while the breasts are squeezed around the penile shaft, creating a tight area for stimulation. In addition, the woman may move her breasts up and down against the penis for an additional stimulation. The act may be carried out face to face, or head to tail, with the woman recommended to be on top if she has smaller breasts. When performed as a non-penetrative sex act, the act is continued until the point of ejaculation for the man.
Mammary intercourse is said by Alex Comfort to produce mutual orgasm in women with sensitive breasts (what Margot Anand terms local orgasms of the breast), and it was one of the nine substitute exercises for penetrative sexual activities, as detailed in the Paradis Charnels of 1903. This act can be combined with fellatio.
Since mammary intercourse is a non-penetrative sex act, the risk of passing a sexually transmitted infection (STI) that requires direct contact between the mucous membranes and pre-ejaculate or semen is greatly reduced. HIV is among the infections that require such direct contact and is therefore very unlikely to be transmitted via mammary intercourse. A study of the condom usage habits of New Zealand's sex workers said that they offered various safe sex alternatives to vaginal sex to clients who refused to wear a condom. One sex worker said that mammary intercourse was one alternative used; mammary intercourse performed by a woman with large breasts felt to the client like penetrative vaginal sex.
Mammary intercourse has sometimes been considered a perversion. Freud, however, considered such extensions of sexual interest to fall within the range of the normal, unless marked out by exclusivity (i.e. the repudiation of all other forms of sexual contact).
Mammary intercourse is known as titty-fucking, titfuck, titjob or boob job in the United States, as well as tit wank or French fuck in the United Kingdom - the latter term dating back to the 1930s; while a more jocular equivalent is a trip down mammary lane.
The Japanese sex industry uses the term paizuri. Paizuri in the Japanese writing system is パイズリ with the first part パイ being from the word おっぱい (pronounced as oppai) which is a slang word for breast, and the second part ズリ being from the word 擦り (pronounced as zuri) which means "rub".