When using in internet conversations or in real-life scenarios whether during AIM, twitter.com, Myspace or other virtual texting messages, this is an abbreviation for "Make Up Your Mind". Just add the statement MUYM. Yesssir!
Gangster #1: Man I'm trying to go to the movies my good pal.
Gangster #2: Oh shit my brother let's come through!
Gangster#1: Ugh, nvm I got errands to utilize.
Gangster #2: Damn my good man, MUYM bro.