Magic: the Gathering slang.
As a verb, it means "to obtain something that is unusually great or unexpected".
As a noun, it means "something unusually great or unexpected".
Often used to describe topdecks or good rares opened: "And he mised an Overrun off the top, that fucking fuck!"
Occasionally humorously used to refer to a non-Magic-related event: "Your girlfriend is so hot -- she's a real mise!"
Alternately spelled "mize".