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, considerfor example:
1) If you need anything, LMK.
2) What're you guys doing tonight? LMK.
The letters LMK itself stands for 'let me know'. If you are talking about the app 'LMK' then it basically an app where you can meet new people, but it also has a bit of tinder in it as well. People generallydon't even use the app as a dating app; they just explore topics such as politics, race, LGBTQ and religion most of the time.