Also known to as slf, a secret life fatty is someone who looks skinny or fit in real life but is so physically unfit that running two feet makes them collapse from exhaustion.
Secret life fatties also tend to consume unhealthy foods on a regular basis, also adding to their lack of physical fitness.
Person A: "You don't need to lose weight. Look at how skinny you are, I bet you could outrun me by miles"
Person B: "Yeah but I am such a secret life fatty. Running from here to that tree a few feet away is a tough feet for me. I know! Let's go get chinese and donuts!"