A sarcastic remark or criticism packaged nicely (or usually not so nicely) into a compliment. A compliment loaded with false sincerity--or sinistercarity.
The "sarcompliment" is a favorite tool of the passive aggressive compliment giver, as once the receiver of compliment detects its sinister purpose, the giver of the compliment can quickly refute the hostility buried within the compliment by saying for example, “I mean it, I really like that shirt. Justin Timberlake wears shirts just like that, and I really like Justin Timberlake!”
(Notice that the giver of the sarcompliment adds more sarcasm in the defense, further putting down the shirt and leaving one wondering, "does he/she really like Justin Timberlake's shirts?" But of course, at least in this example, he/she does not like Justin Timberlake's shirts.
No offense to Justin Timberlake, who does in fact wear really nice shirts. I mean it!