LMK means is "Let Me Know." It can be used differently depending on the context in which it's said. Mostly, when someone says LMK, it refers to getting back to them with relevant information or an update. There are cases where LMK would stand for "Love My Kids" or "Little Miss Know-It-All," but these are not common uses. Thus, when LMK pops up on a text message, the safest bet is to interpret it as "Let Me Know." This is by far the most commonly accepted day-to-day conversation meaning.