demente

demented (dated, clinical)

Su abuelo, ya demente, no reconocía a nadie en la casa. His grandfather, by then demented, no longer recognised anyone in the house.

crazy; mad (colloquial)colloquial

Tu plan es demente: nadie cruza el desierto en bicicleta en agosto. Your plan is insane: nobody crosses the desert by bike in August.

Es un plan demente, pero tal vez funcione. It's a mad plan, but it might work.