A backhanded compliment is nothing you necessarily want to get: it is a compliment that also insults at the same time or saying it the other way around: an insult that masquerades as a compliment.
A backhanded compliment might first fool you into thinking you received a real compliment but it is a deliberate and rather subtle way to disguise an insult. Sometimes backhanded compliments are not intentional but generally the term applies to the intentional use of a disguised insult.
Here are a few examples:
“You are sure smarter than you look.”
“I love you new haircut, it slims your face a lot.”
“Long skirts look good on you, they hide your calves.”
“You drive very well, for a woman.”
“Your son is more handsome than I would have expected.
In each case there is a compliment that immediately gets negated by an implied criticism, namely that a person is stupid, her face is too wide or her legs to thick, she can’t drive or his parents are ugly.
Another term meaning the same thing is left-handed compliment. Traditional the left hand has had sinister connotations and therefore a left-handed compliment is one that is devious.
Not a very politically correct expression now that we don’t discriminate against “lefties” anymore.