2. To cum, jizz or nut on or about somthing.
2. I was fuckin her from behind and decided to jay all over her back.
3. I was fuckin that chick and her pussy smelt so bad that i lost my hard and couldnt jay.
This means they constantly undermine your ideas, statements, actions, etc.
When someone is Jay'ing it up there is a possibility that the are being a total douche or sarcastically supporting your ideas. If someone is Jay'ing it up, call them out on it.
Guy 2: Thats a gay idea bro
Guy 1: Stop Jay'ing it up!