CYA may refer to any of the following:
1.Shorthand for see ya, CYA is commonly used in chat rooms to let users know you are leaving the room. This term is also another way of saying "good bye" to users who may be leaving. Below is an example of how this could be used in chat.
User1: Ok I've G2G
User2: Ok CYA
<User1 has disconnected>
2. Shorthand for cover your a**, CYA may tell someone to watch their back, or to take prudent action to avoid trouble or danger.