noun, feminine/kup sɔ̃bʁ/plural: coupes sombres
light felling, thinning of the trees; drastic cuts
“faire des coupes sombres dans quelque chose” — to make drastic cuts, to make swingeing cuts, to make substantial cutbacks
From woodcutting; the idea being that the forest is still dense and dark, because it was only lightly felled.