One who is considered to be magnum in the sense of ultimate or way cool.
A: Even before she bought us dinner, I always told people your sister was a Nay Nay.
B: Yeah, she is magnum, no doubt.
A: No doubt!
B: So why they call them Greensleeves if they ain't got green sleeves?
A: They ripped 'em off so they could use them for those twirly things.
B: That so magnum.