backdoor compliment

A backdoor compliment is a way of giving somebody a compliment, which can be misread as an insult.

Poppy: I'm whiny as hell, no fun at all.
Dave: I think you are. Fun that is, not whiny - maybe a little, in a nice way.
Poppy: Hey!!! Only I'm allowed to call myself whiny

Dave: Yeah but I said 'in a nice way' so it was a compliment
Poppy: Backdoor compliment!