Fat ninja

When a girl on tinder posts pictures of only her face, and hides the fact that she's fat

Had a date with a really sneaky fat ninja, can't believe I didn't see her second chin in her profile pic!

Fat Ninja

The unsubtle affair. Where two people think they are being sneaky, but everyone knows what is going on.

The act of cheating out in the open, but being oblivious to it.

Joe: "Do Bill and Sue really think no one knows they are hooking up?"

Bob: "I know, right? Pretty Fat Ninja there."