Wiktionary
n. A sexual act in which two males rub each other's penises. vb. 1 (context slang English) To publicly rub one's genitals against someone for sexual gratification, especially without the other's consent or knowledge. 2 (context archaic English) To rub, chafe. 3 (context tanning English) To work leather by rubbing.
Wikipedia
Frot ( slang for frottage; ult. from the French verb frotter, "to rub") is a non-penetrative form of male-male sexual activity that usually involves direct penis-to-penis contact. The term was popularized by gay male activists who disparaged the practice of anal sex, but has since evolved to encompass a variety of preferences for the act, which may or may not imply particular attitudes towards other sexual activities. Owing to its non-penetrative character, frot has the safe sex advantage of minimizing the transmission risk for HIV/AIDS; however, it still carries the risk of skin-to-skin sexually transmitted infections, such as HPV and pubic lice (crabs), both of which can be transmitted even when lesions are not visible.
Usage examples of "frot".
Liqueurs would be passed around and much licking would obtain until our erect nipples, flushed cheeks and moist quims sought further fulfilments, I knowing that I might find myself abed afterwards with Stepmama and Mary while Bertha would frot with Sarah and occasionally bring a cock again to her.
He wears his knickers, petticoats and stockings better now and stands most docile while I frot his prick.
He was as excited as the mage when the box began to make some of the right sounds, as if finding them on its tongue: frot, brot, brat, bret, bread.
They stood, two brown left hands gently frotting the metal monkey pole.