Acronym used to decribe the feeling of waking up on Monday morning. Sort of how TGIF explains the feeling of how wonderful it is to be Friday. OFIM gives the opposite feeling for Monday
This is a term usually reserved for situations where the only solution for avoiding an untimely death is to run for the hills. While this term is sometimes seen in lower case, it is always screamed at the top of ones lungs, usually right before the person behind them is killed/maimed/turned into a newt.
"OMG, is that Manbearpig?"
"OHFUCKRUNWEREGONNADIE!!!"