Dee Michel wrote me a little while ago with the Adj + N phrase diabetic socks, which he found entertaining: how could socks be diabetic? The short answer is that though diabetic is an adjective in this phrase, it functions semantically like a noun, in particular like the noun diabetic ‘someone suffering from diabetes'; diabetic socks are ‘socks for diabetics’. From Wikipedia:
A diabetic sock is a non-binding and non-elasticated sock which is designed so as to not constrict the foot or leg. Typically sufferers of diabetes are the most common users of this type of sock. Diabetes raises the blood sugar level, which can increase the risk of foot ulcers. Diabetic socks are made to be unrestrictive of circulation.
(I am in fact wearing diabetic socks as I write this posting.)
So diabetic here is a type of non-predicating adjective, a type known in the trade as a pseudo-adjective: an Adj in form, but interpreted by reference to a N.
In the case of diabetic, we have not one, but two, pseudo-adjectives — one evoking the noun diabetic (as above), one evoking the noun diabetes (as in diabetic coma ‘coma caused by diabetes’).