An amalgamation between a "munter" (see munter) and a mattress.
Usually a large individual that should know their limits, and comedic for all the wrong reasons.
"Muntress is at it again..."
Australian slang. A matress which someone has munted (vomited) on. Muntress can also include sweat, piss and pool water.
Omg, Em just munted on that matress!
Someone put that muntress outside, it reeks!