In AD&D, thieves and assassins of high enough levels may backstab their enemies, thus multiplying the damage they do. This is done -- surprisingly enough -- by standing behind them, and stabbing. Such a blow is presumably harder to dodge or block (hence, indeed, the other uses of the word). As AD&D's combat system is disgustingly over-simplified, there's no easier way to put this form of stabbing into the rules than adding this hack to increase damage done.
In Nethack, all character classes can backstab. However, palladins had best ensure they don't do this, as their gods don't tend to see it as commensurate with their vows. Note that the NetHack user interface (generally in glorious 80x25 ASCIIvision) gives no direct feedback whatsoever on which way your foes are facing! This only serves to make things more interesting.