LMO
LMO has 2 meanings
Let's make out
If you are attracted to a person and want to kiss someone, you can send them "LMO," which stands for "let's make out." However, be careful who you send it to because it is an incredibly forward prompt that may scare people off.
Most people use LMO when texting or messaging online in a private chat. For example, a bf may text his gf during school lunch, "Meet you under the bleachers in 5 mins. LMO."
While people typically use it with people they know well, some creepers may send it via DMs on Twitter or Tinder. If you want to shake off unwanted advances from creepy guys, simply reply with "Ew, gross" or "GTFO."
Example
I haven't seen you in almost an entire day! LMO soon.
LMO kissy face
Means "Leave me a message;" used when someone is away from his or her instant messaging program to let people know they can still leave a message.