Literally from the Spanish "without seeds," or so I'm told. Sinsemilla is the best kind of marijuana, since the plant it came from never stopped producing resin until it was harvested. All really good bud is sinsemilla, although it is theoretically possible I suppose to have sinsemilla that is not great bud(but I haven't seen any). In addition to the obvious advantage that sinse gets you higher, you don't have to pick out seeds like you do with lesser herb.
Sinse is obviously the most expensive type of bud you can buy, usually running around $50 an eighth. It's definitely worth it though, because sinse is IMNSHO the best recreational drug there is. While you might smoke all night and just get a buzzy kind of headache from schwag you can take one hit of sinse and be stoned for hours. Sinse is also referred to as kind, dank, or chronic. If you've never had any, go out and get yourself some. If everyone smoked sinse the world would be a better place.