When you compliment someone, but it first sounds like an insult, (and probably is). Called a ninja compliment because the compliment is in there, but it's as hard to find as a ninja.
Janet: "You look great now you're not so fat. Before you looked really bad"
Jane: "So, she said I looked really bad before, but I'm better now I'm not so fat."
Jill: "Wow, so a ninja compliment then...."