Chaol dimples are simply what peasants would refer to as "Venus Dimples". Dimples above your backside, special for some reasons. Lord Westfall referred to them as if God had pressed his thumbs over your skin and marked you this way. So you're Westfall gifted, Westfall appreciated but most importantly... Westfall noticed
"Indentations in her lower back, as if some God had pressed his thumbs there" Chaol over Chaol dimples