it is appropriate to call someone a mong biff when they have done something utterly stupid and/or idiotic.

it can involve more than one person, the person who notices the act of specialness will say "Mong" and it is open for anyone else to add "Biff"

bringing more humiliation to the person who experience the special moment
Chris walks into a lamp-post
Tori cries out "MONG..."
Greg adds "BIFF"

therefore Chris is a mong biff
by jabba dabba doodlely June 30, 2010
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Dishevelled, rakish, disorderly, or unkempt. Used of hair, neck ware, or one's general disposition. Opposite of "rectilinear-biff." Origin unknown, but possibly derived from a misunderstanding of "skew-wiff." Usage peculiar to Brasenose College, Oxford.
"My word, Terence, with your tie at that angle, and your obvious bed-head, you're looking unusually skew-biff!"

"Don't worry: I get myself rectilinear in no time."
by Mercurius Oxoniensis June 7, 2009
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A biff jangler is the term used to describe an overhanging gut that covers their own genitals (biff).
Fuck me Steve, you see the size of that birds biff jangler? Looks like Pop and Fresh had a fight with a microwave! Vile
by TinaAndCrumps February 9, 2019
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da biff: Simple, bogan form of "The Biff." Often extended into a very long sylable, to be uttered as a extended groan, to point out the various uncultured tendencies of those who cry out, "Bring back the biff."

Also used to refer to crappy, mingey players.
YO, DA BIFF. HARDCORE, BRO. HARDCORE.

DAAAAAAAAAAAAAAAAAA BIIIIIIIIIIIIIIIIIIIIFFFFFFF
by Twittah May 26, 2009
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A Rather large part of a womans genitals, can be used to imply that a woman has had many sexual partners.
"That girl i took home last night had a bucket biff, i thought i was gonna fall in it!"
by Hailee Jayde August 27, 2007
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When playing a game of Halo or Halo 2, you biff (smack them) in the back with the flag, killing them on contact.
You assassinated New001.

"OOOH you see that sucka?? That was a flag biff you freaking noob"
by Italian Biggun November 21, 2004
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