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1.
short for "most definitely" (contrary to popular belief it is NOT mo def, and whoever said that is fucking retarded), used because saying most definitely sounds gay and mo defs sounds cool
Kid 1: dude that girl is so hot
Kid 2: most definitely
Kid 1: wtf
Kid 2: Uhh I mean mo defs
Kid 1: true dat
Kid 3: mo def
Kids 1 and 2: ...
Kid 1: shut up dumbass
by disneysucks December 01, 2006
 
2.
Most definetly
"katie are you going to the mall?" "mo def!"
by Alia June 09, 2003
 
3.
A shortened version of most definitely. This phrase is best used in drunken converstations and text messaging.
Girl 1: OMG, that girl is totally wasted.
Girl 2: Modef.
by JessiB July 21, 2008