Also see sentences for:brainless,childish,defective,fatuous,foolish,idiotic,imbecile. Definition of silly: silly, sil’i, adj. simple: harmless: foolish: witless: imprudent: absurd: stupid. | n. a silly person. | adv. sill’ily. | ns. sill’iness; sill’y-how, a caul. (0) ...