hammerspace is that kooky extradimensional pocket where anime / manga characters (most often, female) reach in (usually behind their back) and pull out shoujo hammers to whallop someone (usually male) good for saying / doing / being something stupid. usually this offensive strike is accompanied with the phrase, "baka!" the hammer is the default blunt object to strike someone with, provided there isn't something sufficiently heavy in the vicinity or kicking the offending party into the horizon isn't a viable option. the origin of this concept is most likely found in fanfic (or fanfiction) written by English-speaking authors in an effort to explain this abberation of the laws of physics.

examples of this are demonstrated wonderfully in such anime as ranma 1/2 and in most manga where there is light, violent humor (again, ranma 1/2, sorceror hunters, etc). look at laws of anime for other such kooky things.