Some of us instant message (IM) at work. In many workplaces, IMing isn't encouraged (and so it shouldn't be, because it's rather disruptive to one's work)... so we do it in secrecy. But if you're in a conversation with someone while your boss comes by to talk to you, the last thing you want is for 4 incoming message tabs blinking away in your Start Menu bar, which can give a bad impression.
What "DMM" really means is: "As of this message, don't send anything to me until I message you again. I'll explain later." In other words, "NO MESSAGES". It's usually not enough to just say "hold on" or "brb", because everyone's first reaction to that is to send a message "OKAY", which starts up a blinking message tab. No help there. When you receive a DMM/dmm message, just leave the person alone until the coast is clear. So, add this to your IMing dictionary.
Term is often used in automotive and homeowner forums to describe troubleshooting procedures.