It simply means "
let me know." Or get back to me with answer.
If it already formally exists, I don't know, but I started using it in my e-mails in the '90s for
convenience. Today, someone responded "OH!!!!!!" after I replied to his "what's the meaning of that"
e-mail. So, there you have it.
In
e-mail commonly, consider for example:
1) If you need anything, LMK.
2)
What're you guys doing tonight? LMK.
