Costeau (no idea if that its correct spelling as never seen it spelt) is argot for strong. I used fort the other day and a nice French lady in the market queue agreed, but used costeau, and said it was an argot word. All done in French of course, though she knew I wasn't French (they always seem to know, even though they all say as well that I speak good French!) So used to those who don't try, perhaps!!
ex W1, via 47 and 11 and now [just] in 34, equidistant from Carcassonne, Narbonne and Béziers, where I hope we'll finally stay!!