Is "laughable" a noun or adjective?

Question

My English professor corrected me when I wrote the following sentence: "What's so laughable?". Obviously I wanted to convey that something seemed laughable and I wanted to know what, but he answered that "laughable" is a noun, and he gave me this option: "What's something that you can laugh at?" I'm totally bewildered because in the Oxford Learner's Dictionary it appears only as and adjective. Thank you in advance.

Answer 1

Your English professor is wrong, and you are right: laughable is an adjective. It might be employed as a noun (though I have never seen this), but if someone wrote or spoke of the laughable it would not designate a laughable entity (person, thing, event) but the category or set of all entities which are laughable.

I think, too, that your English professor misunderstands your question "What's so laughable?" If you are employing this in its ordinary, entirely idiomatic sense you are not, as your professor's paraphrase suggests, asking your hearer to name some laughable entity—you are asking your hearer to justify the characterization of a particular entity as laughable.