A compliment that also insults or puts down at the same time.
They gave me a backhanded compliment when they said I was smart for a girl.
Back-handed is synonymous with left-handed. For example in tennis, a backhand stroke is a strike by a right-handed player from the left side of the body.
The left side of the body has always been deemed sinister. The Latin word for left is sinestra. Hence, back-handed means round-about, indirect, or devious.
In America, left handed people were once considered suspect and untrustworthy. Until the 1960's many people who were naturally left handed were encouraged as children to use their right hand for tasks and were taught to be righthanded.