left-handed compliment, a

An insulting or negative comment disguised as praise. She said my new pants really make my legs look much slimmer. What a left-handed compliment!

Also, backhanded compliment. An insult in the guise of an expression of praise. For example, She said she liked my hair, but it turned out to be a left-handed compliment when she asked how long I'd been dyeing it . This expression uses left-handed in the sense of "questionable or doubtful," a usage dating from about 1600.

An expression of praise or admiration that is actually a faintly disguised insult or reproach. The association of lefthandedness with ambiguity or doubtfulness may come from the practice of the morganatic marriage ceremony (between royalty and a commoner who renounces all claim to the spouse’s title and property); in it the groom gave the bride his left hand instead of the right hand used in conventional marriage ceremonies.