Shorthand for 'teh shit', meaning the elite stuff, or the superior substance. Often used in IM services such as MSN or even MMORPG games like World Of Warcraft. The word 'teh' is a MMO l33t word for 'the', when people who type too quickly spell the word wrong. Can also be used as an adjective.
Damn that weed was tesh fool!
Hey, I just dinged my char 2 60, teh nu moovs r tesh!
That is some tesh porn.