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2.
LMK
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 03, 2006
 
1.
LMK
Let me know
What do you think? LMK...
by Anonymous October 01, 2003
 
3.
LMK
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 16, 2012
 
4.
LMK
1. Launched my Kitten

2. Lick my Klit
What did you do today?
LMK!

Hey could you LMK?
by Filledebatteur4 April 17, 2011
 
5.
LMK
1. Let Me Know
2. Lick My Knuts
"Just LMK if I can help you out"
-or-
"Just LMK if that will help"
by Anonymous September 30, 2003
 
6.
LMK
Lets make Kids Let's Make Kids
Username: I just got a million dollars.

James: LMK! LMK!
by James Purvis April 22, 2008