Muffing is a sexual technique for manually stimulating the inguinal canals, interior passages within the groin located behind the scrotum and testicles. In fact, the testicles descend from the inguinal canals, and trans women sometimes tuck their testicles into them, allowing them to "tuck" their dicks. Although the inguinal canals are covered in flesh, they can be penetrated from the outside, providing pleasure to the receiver. Although muffing is pleasurable, most people say it is best as a form of sensation play or foreplay, not a path of orgasm.
While the term was originally described to give pleasure to trans women, men could also enjoy muffing. However, for them it won’t be used as a substitute for the fingering that people with vaginas enjoy.