This acronym is used to say 'contact me', 'text me', 'phone me' or otherwise 'reach me to follow up on this'. It is a modern shorthand way to invite a person to communicate with you further. HMU 'hits me up' used to mean 'that person asked me for something', as in 'David hits me up for a small loan every Friday night'.
This expression has now morphed to mean 'communicate with me further'. HMU can be spelled in all lowercase as 'hmu' Both uppercase and lowercase versions mean the same thing, and you are welcome to use either. Just avoid typing entire sentences in uppercase, as that is considered rude shouting online.
