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.
1) If you need anything, LMK.
2) What're you guys doing tonight? LMK.
2. Lick My Knuts
"Just LMK if that will help"