What does it mean?
Originally found in the 1990 lyrics from MC Hammer's song "Can't Touch This" Hammer sings "Break it down, Stop - Hammertime!" right before he breaks into a dance solo.
According to Yahoo answers, the saying has evolved to mean that "it's time to get busy" or "do something like get to work."
An entry on Urban Dictionary says that Hammer Time is one minute every day that is dedicated to awesomeness. "The official time of Hammer Time has been set by the internet community to 11:42 AM."
My favourite, out of all the explanations I collected, is that this phrase is commonly used right before hitting someone with the Gravity Hammer in Halo 3. Now I want to play Halo so I can get some Hammer Time!