What Does “LMK” Mean, and How Do You Use It? Let Me Know LMK stands for “Let me know.” This initialism is used to tell someone that they should inform you of something in the future. For example, you could tell someone who is currently sick to “LMK if your fever goes down” or “LMK if you need anything.” The acronym often implies that another conversation will take place in the future, likely when the other person replies. It is mostly used in texts and emails instead of real-life conversations. When speaking to someone IRL, it’s best to use the full phrase “Let me know.” You can use this acronym in both the uppercase LMK and the lowercase lmk, as both are equally common.