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.
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