When used in any type of text messaging system, HMU is "Hit Me Up." It means that they want you to get back to them - another way of saying "contact me", "text me", "phone me" or otherwise "reach me to follow up on this".
With hundreds of millions of people texting regularly, it's no wonder you've seen a number cryptic looking codes. Acronyms have always been an integral part of computer culture, and they have since spawned a new language on the Internet.
Check out the actual definition of an acronym and text shorthand on NetLingo.com.
Speaking of text messages, remember when news broke that Tyga was allegedly messaging with a 14-year-old but it turned out that she was lying about her age?
Well, its being reported that this same high school aged girl, Molly O’Malia, has been caught messaging another rapper and lying about her age yet again! Get more scoop at TheShadeRoom.com.