The act of looking at your opponents screen or playbook in a video game to gain an advantage.
In football video games, paddle sniffing is achieved by trying to figure out your opponents play they're going to pick by looking at your playbook. In split screen games like Halo or Mario Kart, one paddle sniffs by looking at the action on their screen to be able to find them to fuck their shit up.
The ethics of Paddle Sniffing is quite controversial. Some argue that anything is on the screen is fair game to use to their advantage. Yet the majority opinion is that Paddle Sniffing is for pussies and is used only if the Paddle Sniffer sucks at the game and that's the only way to be able to win.
Paddle Sniffing is often times accompanied by extreme douche-baggery, whereas, major studs are usually known to never paddle sniff.
victim: You knew I was about to run the option, you Paddle Sniffing mother fucker. Grow up and run your defense like you know anything about football.
paddle sniffing dickhead: Hah, I totally paddle sniffed you being in that corner and that's how knew to snipe you there.