n. 25B6; adjective
don't be so silly : FOOLISH, stupid, unintelligent, idiotic, brainless, mindless, witless, imbecilic, doltish; imprudent, thoughtless, rash, reckless, foolhardy, irresponsible; mad, scatterbrained, feather-brained; frivolous, giddy, inane, immature, childish, puerile, empty-headed; informal crazy, dotty, scatty, loopy, screwy, thick, thickheaded, birdbrained, pea-brained, dopey, dim, dim-witted, half-witted, dippy, blockheaded, boneheaded, lamebrained; Brit. informal daft, divvy; N. Amer. informal chowderheaded; dated tomfool.
that was a silly thing to do : UNWISE, imprudent, thoughtless, foolish, stupid, idiotic, senseless, mindless; rash, reckless, foolhardy, irresponsible, injudicious, misguided, irrational; informal crazy; Brit. informal daft.
he would brood about silly things : TRIVIAL, trifling, frivolous, footling, petty, small, insignificant, unimportant; informal piffling, piddling; N. Amer. informal small-bore.
he drank himself silly : SENSELESS, insensible, unconscious, stupid, into a stupor, into senselessness, stupefied.
sensible.
25B6; noun (informal) you are a silly! : See fool noun sense 1.