a sneak
When you're with someone and you're like, 'Hey, I got a new friend,' but you're cheating on your other friend.
He said, 'I got a new friend,' but I knew he was cheating on me.
She said, 'I got a new friend,' and I saw her talking to him in the hallway.
He was with a new friend and I was like, 'You're cheating on me.'