"Like a shag on a rock" - Literally, it means like having sex on a rock... it means to be exposed, to stand out (badly).
I feel a need to make a small correction here. A "Shag" is a water bird in the Cormorant family - often seen with their wings extended in the sun, standing on rocks.
It still means to be exposed though.
Ha, even native speakers get it wrong! Thanks, I will fix that!