Backstab is a thief skill where you sneak up on your opponent and stab them in the back with a dagger or other piercing weapon. After backstabbing, the attacker is lagged for a short time.
Note that backstab is not totally effective on many creatures. Skeletal creatures are nearly immune, while creatures that are injured (HP at 30% to 90%) are generally much more aware and harder to sneak up on. Creatures who hav 0-29% hp, or 91-100% hp, are far more likely to be successfully stabbed. In PK, injured targets (30-90% HP) have about a 50% chance of being backstabbed as compared to a far lower (About 6%) for mobs.
Some of the most exciting moments in the life of a backstabber is when he or she gets a freak stab. Freak stabs take the damage that would normally have been dealt, and multiply it by a random power of 2 between 1 and 7. Any backstab multiplied by 8 or higher will be broadcast to the entire mud. Freak stabs on the higher end of the scale (32) are fairly rare, while freak 8 and freak 16 happen once or twice a day. For stabs above freak 32, the lucky thief will receive a practice. For any freak stab, he will also gain a higher-than-normal amount of xp. Many previous freak backstabs can be seen on board 10, the hall of trophies.