What is "hammerspace"

n. (context humorous English) A notional extradimensional space in which large objects can be stored so that they may be quickly retrieved at any time.

Hammerspace (also known as malletspace) is a fan-envisioned extradimensional, instantly accessible storage area in fiction, which is used to explain how animated, comic, and game characters can produce objects out of thin air.

This phenomenon has existed in animation for years, dating back to early Warner Bros.' Looney Tunes/ Merrie Melodies animated cartoons.