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1.
Similar to TGIF ( Thank God It's Friday ), meaning Oh No It's Monday
*alarm clock*

student: ughh...ONIM.
by spidiee August 23, 2009
 
2.
An acronym standing for "Oh No, It's Monday".

Used to express the discontentment and dread one feels for Monday, the beginning of another tedious week.
Tito: "YES! let's get this weekend started!"
Todd: "You idiot, you've been passed out for 3 days."
Tito: "wait, what? O.N.I.M.!"
by ABDICKOX November 15, 2010