Mylyb - acronym for the phrase "miss you, love you, bye." It is a more simplistic way of saying goodbye to someone in a text, rather than typing out the full message.
Will: I'll talk to you later, babe.
Val: Ok, mylyb!
Say you are texting someone, while stopped at a stoplight, and the light turns green while you are saying goodbye to them, simply type "mylyb" and they can interpret that as you being in a hurry, but still caring enough to not leave them in the middle of a conversation.