hammer
ham·mer  n. 

A particularity difficult and gnarly trick performed on a skateboard, the emphasis being on the sickness, as opposed to grace or flair of its execution. Usually down or over a large obstacle, more often than not, sets of stairs and handrails, often switch and at great risk to the skateboardist.

Origins: Popularised and derived from "Hammertime" (MC Hammer), to Hammer (verb), and then to noun, e.g. a Hammer is the product of said ‘Hammertime’ (to ‘go off’, ‘come up’, ‘kill it’ or ‘bust out’).

This meaning of Hammer is promoted most vigorously by Jim Greco. Although once a staple of the 'Hesh' community, Hammers now transcend all hesh/fresh barriers, as the documentation of such Hammers is closely linked with making mo' money though incentives and sponsorship, and translates seamlessly into the philosophy of mainstream rap.
"I got a mad hammer filmed for my part, i'm so ill I make me sick"

"yeah he can throw mad hammers, whatever, his steez mings; pushes like a buggered penguin having an epileptic fit"