The abbreviation ‘sus’ has two meanings, ‘suspect’ and ‘suspicious’.
How it is taken all depends on the sentence it is used in. When you are called ‘sus’ by someone they are actually calling you fake, false, and/or shady. This happens if you appear to be hiding something causing you to look suspicious or perform acts that make you appear suspicious. Or simply being in the wrong place at the wrong time, making you suspect as to what happened.
Texter 1: Did you see her shoes?
Texter 2: Yeah they were covered in mud.
Texter 1: And she said she didn’t go out.
Texter 2: IKR very sus.