First-Aid is a skill used to restore hp, undo status ailments, and repair gorging damage. It is unique in the latter regard, which makes finding the Scroll of First-Aid a must before confronting a creature that gorges.
First-Aid is administered by issuing the First-Aid Command, "1-A", which creatures a miniature adventurer and a challenging six-level "inside-your-dwarf" dungeon. The mini-you has all of your abilities and full health; guiding him to the sixth level of the You Dungeon gives him the opportunity to heal the wound. If he dies, then the dwarf dies as well.
When you come out of First-Aid, however, all of your mana points are spent. Because of this, and the high risk of failure, most players reserve First-Aid for gorging damage and do not waste it on a trivial matter like a tummyache.
It is possible, in the current version, for your mini-you to jump into the game world and replace your dwarf. See: Mini-Exploit.