The type of sweater a guy puts on when he's trying to impress the chick he's on a date with. It's the only nice piece of clothing he owns and you'll see him wearing it in pretty much every picture taken of him on a date.
Can also be a useful tool when you're trying to confirm if two people are dating.
Sally: "Jenny, I told Tony I just want to be friends, but he keeps wearing this nice sweater when he comes over, even if it's just to watch TV."
Jenny: "Oh yeah, he's definitely interested. That's his date sweater. But I wouldn't date him, Sally...he's a total meathead."