It's not as sure as I was led to believe. Meyer-Lübke derives Fr barbacane (as well as It & Sp barbacana, Pg barbacã) from a Provençal word that is from Arabic barbahhane 'wall'. He also states that "the word first comes to light in a document from Marseille in the 12th century. Whether the its entry point was in Spain or SW France, one cannot say." A third, but less likely origin, is from Latin barba canis 'dog's beard'.