Moronic means so stupid it makes you groan. A moronic joke, a moronic thing to attempt with a ladder.
It is an insult and a blunt one, so it lands harder than silly or daft. You can call an idea moronic, a rule moronic, a whole plan moronic — and applying it to a person is stronger still. The noun moron is blunter again. The word began life a century ago as a medical classification, which is exactly why it feels cruel now, and worth knowing before you aim it at anybody.
From moron + -ic.
Etymology from Wiktionary, CC BY-SA