You can look it up. It may seem harsh, but calling your father-in-law a fool makes you liable to the fires of hell. After all, he may have been conned by the lies and empty promises, duped by a charismatic scam artist, or confused by the self-contradictory rhetoric. He may have a disability that diminishes his powers of reason and understanding. Let's have a little empathy here, folks.
But. Nowhere in the Book does it say that you can't call him an asshole. You can reference your Cruden's Concordance (or Strong's, for that matter) and you won't find a single citation for the word.
Not that I'm trying to split hairs here; there's a difference.
You see, being an asshole means willfully ignoring the lie and turning a blind eye to what is acceptable in a rational and humane society, perhaps even in a hope for personal advantage over the wider community.
So. Asshole okay, fool not okay. Got it?
Of course, there's "Judge not, lest ye be judged," too. Not easy to figure this stuff out.