It means unattractive and objectionable. It’s a word of no great age: it only began to appear at the end of the nineteenth century. It’s still moderately uncommon, though you will spot it in the more up-market newspapers or in the work of writers with more power to their pens than most of us. A couple of examples:
Anyway, with red or without it, you should now have a reasonable image of Leon Kossoff’s Seated Woman No 2, by some stretch the muddiest and most rebarbative painting I’ve seen this year.
The Independent, 16 Dec. 2011.
In Mortimer’s view, many might indeed find Lady Chatterley’s Lover and the practice of riding to hounds rebarbative, but that was no reason for the authorities to prevent people who wanted to experience both freely from doing so.
Washington Times, 26 Nov. 2011.
The touch of weirdness in this word comes not from its unusualness but from its history. If the middle bit of rebarbative makes you think of barbers, you’re on the right track — the ultimate source is Latin barba, beard. Rebarbative came into English from the French rébarbatif with the same sense. This has been in French since the fourteenth century — it derives from the verb se rebarber, which referred to two men squaring up face to face, beard to beard, in close-quartered and hairy aggressiveness.