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1.
i.e. mazel tov, lit. meaning "good fortune" in Hebrew and Yiddish. Jewish people use this expression to congratulate each other when something good happens, meaning something like good for you, well done, or congratulations.

A: I just passed my driving test.
B: Muzzletoff!
by semicolumn January 26, 2009
 
2.
an alternative to the word "messed up"
"dude that things she did last night was muzzle toff"
other guy "ya dude, dont give innocent mormon girls liquor"
by cursorkidjr June 29, 2011