The meaning of lmk is “let me know”.
It is mostly used online, in chats and emails, although it is almost exclusive to casual conversations.
Like many abbreviations, using “lmk” in a professional environment may reflect badly on you.
The contracted form of “let me know” was conceived sometime in the 1990’s, when the spread of technology such as the internet and later SMS had swept over the lives of people.
Similar to other abbreviations, it rapidly turned into a commonly recognized phrase, thanks to its widespread use among both young and adult users.
The growth of the web and the accumulating information about slang expressions had led to the further spread of “lmk”.
It was first defined on Urban Dictionary in 2003, and it later appeared on several other sites’ definitions about abbreviations.