Underground

If a modification artist is said to be underground, it generally means that they are working without recognition from their "governing community."

An underground cutter is a surgical practitioner working without the approval of the medical community. A slight variation on that would be the underground doctor who is recognized by the medical community, but also does work "underground"—typically this includes doctors willing to quietly perform subincisions, castrations, amputations, and so on, with relatively few questions asked. Often, these procedures take place outside of their mainstream studio (ie. in a hotel room, or a medical fetish club), or often in a studio in a country like Mexico with less restrictions on doctors' actions.

In terms of piercing or tattooing, calling someone an underground artist usually means that they're not sanctioned as qualified by any studio and are working out of their home (or out of a bag).