"repellent, unattractive," 1885, from French rébarbatif (14c.), from barbe "beard," from Latin barba (see barb (n.)). The usual theory is that it refers to the itchy, irritating quality of a beard.
Entries linking to rebarbative
barb n.
late 14c., "barb of an arrow," from Old French barbe "beard, beard-like appendage" (11c.), from Latin barba "beard," from Proto-Italic *farfa- "beard," which might be from a common PIE root *bhardhā- "beard" (source also of Old Church Slavonic brada, Russia boroda, Lithuanian barzda, Old Prussian bordus), but according to de Vaan the vowel "rather points to a non-IE borrowing into the European languages."