avoir une faim de loup

To be ravenously hungry

Wolves have long stood in for a huge appetite, so this just means starving, properly starving.

Sers-moi une grosse assiette, j'ai une faim de loup.

Give me a big plate, I'm ravenous.

It builds on 'manger comme un loup' (to eat like a wolf), a phrase going back to the 17th century, though this exact wording only turns up from the late 19th century onward.