hammerspace is that kooky extra
dimensional 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.