A social group based off the word lethargic, its members mostly tired slackers with low fashion sense, a tendency to play video games and magic the gathering but not at a impressive level. The construction of the word is based off of "emo" being short for "emotional".
A leth is that guy you know that plays halo with you pretty often, but never wins. A leth doesn't fall asleep in school, it just always looks like they will. A leth always was interested in playing World of Warcraft but never go around to it. A leth is somewhere between a stoner and a nerd, without the illegal or anti-social characteristics of either.