A cut made under something. In the body modification "scene," the something is the penis. Specifically, a subincision refers to a slit being cut along the bottom of the penis from the urethra to whatever part of the shaft the person desires, for that filleted look and having a sprinkler, rather than a stream, of urine (known as the dreaded sprinkler-head effect). Sometimes this happens accidentally because of a Prince Albert getting yanked on; this is called a meatotomy.