Originally coined by Mira Bellwether (author of Fucking Trans Women), the term muffing refers to the penetration of the inguinal canals on a male-bodied person. So what are these inguinal canals and why in the heck would you want to penetrate them? It's not as crazy, kinky or scary as it sounds!
Where are those canals?:
The inguinal canals are located in the groin area, try moving your (or your partner's) testicle to the side and rubbing around the area behind the scrotum. You'll find a soft but tight hole on each side. That's the inguinal canals, or rather, the openings called the subcutaneous inguinal rings! Testicles hide in these canals before puberty and drop out fully into the scrotum during early puberty, often referred to as when your "balls drop". Most transwomen and cross-dressing men will be familiar with where these canals are, since they play a role in proper "tucking" (hiding the male genitals so a bulge can't be seen) but they probably don't know that these can be used for pleasure!
Here's an illustration of where they're located if you're having a hard time visualizing it:
From Gray's Anatomy
How do you play with them?:
Once you find them, don't go trying to stick a few whole fingers in right away, the first time you play with them it may be a bit painful. Just like anal sex, start off small and slow. Try trimming the nail on your pinky finger and playing around. Massage the area and put a little pressure on the hole, slowly pushing in a bit more each time. Your finger may slowly disappear into the hole or "pop" right in, depending on your anatomy. Once you've gotten use to your pinky, work up to larger fingers, multiple fingers, and eventually thin toys! I've even heard of someone penetrating them with normal penis-sized toys! That's not something to expect right away of course. Once you've gotten them loosened up a bit, try having your partner integrate muffing into oral play, with a little warm up, your partner may be able to penetrate them with their tongue!
Why play with them?:
There are 4 major nerves in the groin area, all of them bring sexual pleasure and all deserve stimulation! The I nerve (or ilioinguinal nerve) runs through the inguinal canals, massaging, stroking and playing with your I nerve will also excite the G nerve (or genitofemoral nerve) and generally arouse you. The first time you stimulate them, they may be overly sensitive (they aren't use to being played with yet!) so don't get turned off if the first time isn't the best. Just like discovering your prostate and anal nerves, these are pleasures that can really enhance sex (or your solo play), just make sure to not hurt yourself by over-doing it! The I nerve isn't often directly stimulated with "normal" play. This is a whole new sensation to discover, especially if you're looking for new things to add to your sex life or solo sessions.
Anyone with a biologically male body can muff (and be muffed)! As long as you have inguinal canals, you can (and should!) try muffing. This isn't something reserved for kinky homosexuals or super hard BDSM freaks, this is something anyone able can try and enjoy. Some transwomen enjoy muffing as a substitute for vaginal penetration, so if you have a lovely translady at home, be sure to introduce the idea to her! Don't be scared off just because it's uncommon, this can be your little pleasure-secret, or something special to share!