The Martial arts skill is used when attacking or kicking enemies while wielding nothing. Monks and samurai possess this skill, superior to the bare hands skill. The base damage for a martial arts attack is d4. Only 75% of all hits get the damage bonus from this skill.
The samurai has recourse to much better attacks than martial arts, but the extra damage compared to bare hands can come in handy against acid blobs and other weapon-damaging monsters. Carrying some form of ranged attack will greatly reduce your need to resort to unarmed combat, and even a monk can benefit from having projectiles at his disposal.
Attacking with martial arts causes silver damage if you are wearing a silver ring and no gloves, similar to any wielded silver object. Blessed gloves or boots (when kicking) cause +d4 blessed damage against demons and the undead. A ring of increase damage is a worthwhile possession for a monk who loses out on benefits of weapon enchantment, and these can be eaten if appropriately polymorphed and the ring is of the appropriate type (metal for metallivores, wooden for a gelatinous cube). The skill level for martial arts can reach Grand Master with large bonuses eventually accumulating.
Since the damage bonus is only applied to 75% of hits, the average damage is not 2.5 + bonus, but 2.5 + 0.75*bonus. This makes the +9 bonus for Grand Master skill slightly less formidable than it might seem, but still very respectable.
Note that both roles capable of martial arts start with Basic skill, and no roles can advance to Grand Master in bare hands, so two entries in the table are not actually possible.
Martial arts skill
- Bare-hands or martial arts skill only grant their damage bonus if the d2 or d4 base damage is at least 2. See uhitm.c, line 438 and uhitm.c, line 904
- The bonus would be added by weapon_hit_bonus in weapon.c, but this is never called because uhitm.c, line 283 or uhitm.c, line 1957 checks if you're wielding something.
- uhitm.c in NetHack 3.4.3, line 972