What is Lmk?
1.
Let me know
What do you think? LMK...
2.
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.
See
3.
Lets make Kids Let's
Username: I just got a million dollars.
James: LMK! LMK!
See
4.
1. Let Me Know
2. Lick My Knuts
"Just LMK if I can help you out"
-or-
"Just LMK if that will help"