The Collaborative International Dictionary of English v.0.48:

MASH \MASH\, MASH \M*A*S*H\, n. (Mil.)
   An abbreviation for Mobile Army Surgical Hospital,
   consisting of the equipment and personnel required to perform
   emergency operations on injured soldiers, located in tents
   near the front lines of combat; as, he worked in the 25th
   MASH.
   [PJC]