A broad-spectrum word, but the general sense is something that makes you flinch and possibly shy away. Adopted in the early 1890s from French rébarbatif, repellent, disagreeable, from Middle French rebarber, to oppose, to stand up to, from the phrase se rebarber, to stand face-to-face, or literally, to stand beard-to-beard, from barbe, beard from Latin barba, beard. A vivid image, that. Makes me think of two Spanish conquistadors chest to chest in their plate mail with full beards a-flush, staring each other down.
After four weeks of not shaving, my rebarbative stubble has finally reached the attractively scruffy stage.
---L, true sentence.