The term "hmu" is an abbreviation commonly used in text messaging and online communication, which stands for "hit me up." It is an informal way of inviting someone to contact you or to hang out. When someone says "hmu," they are signaling that they are open to conversation or socializing whenever the other person is interested.
The phrase "hit me up" originates from African American Vernacular English (AAVE) and has been adopted into mainstream English, particularly within youthful or informal settings. The abbreviation "hmu" emerged with the rise of text messaging and social media, where brevity is often preferred.