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.
by Lee Djangmah April 3, 2006
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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 generally don't even use the app as a dating app; they just explore topics such as politics, race, LGBTQ and religion most of the time.
2 lovebirds messaging on LMK

"Add me on snapchat, baby cakes

"OK, LMK if you added me.

Hehehhe get the pun?
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LMK what you think about us getting married with all this massive dreams coming true for you my dear
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Means Lick My Kitten, Kitten being slang for the female genetalia, this term would be used in the same context by a female that a male world use the phrase "Suck my dick" in
Girl: LMK
Guy: what's that mean
Girl: lick my kitten
by Random739104638 July 17, 2012
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1. Let Me Know
2. Lick My Knuts
"Just LMK if I can help you out"
"Just LMK if that will help"
by Anonymous September 30, 2003
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