Foolish
Foolish, unintelligent, brainless, witless, imbecile, imprudent, rash, foolhardy, mad, unstable, feather-brained, frivolous, fatuous, immature, puerile, empty-headed, slow-witted, crazy, screwy, brain-dead, thick, birdbrained, pinheaded, dim, dippy, fat-headed, boneheaded, chuckleheaded, wooden-headed, damfool, dotty, divvy, glaikit, chowderheaded, dotish, unwise, inadvisable, ill-considered, inappropriate, irrational, hare-brained, ridiculous, laughable, farcical, asinine, stupid, idiotic, mindless, imbecilic, doltish, thoughtless, reckless, irresponsible, erratic, scatterbrained, flighty, giddy, inane, childish, half-baked, half-witted, weak-minded, loopy, soft, cretinous, thickheaded, pea-brained, dopey, dim-witted, pie-faced, blockheaded, lamebrained, dunderheaded, muttonheaded, daft, scatty, dappy, dumb-ass, dof, tomfool, senseless, injudicious, misguided, illogical, unreasonable, absurd, ludicrous, risible, and preposterous.
Trivial
Trivial, frivolous, petty, small, minor, unimportant, of little account, piddling, trifling, footling, niggling, slight, insignificant, inconsequential, piffling, and small-bore.
Senseless
Senseless, unconscious, dopey, into oblivion, into a daze, dazed, stupefied, muzzy, insensible, stupid, into a stupor, into senselessness, numb, stunned, and groggy.
Nincompoop
Nincompoop, simpleton, ninny, dope, dummy, goon, fathead, chucklehead, lamebrain, pea-brain, thickhead, pinhead, birdbrain, noodle, silly billy, nit, twit, twerp, sumph, balloon, boob, goofball, goofus, dumbhead, lummox, simp, coot, poop, dingleberry, dill, galah, bogan, boofhead, muttonhead, clodpole, mooncalf, dunce, nitwit, dimwit, dumbo, chump, jackass, bonehead, knucklehead, clod, pudding-head, wooden-head, airhead, scatterbrain, donkey, stupe, clot, berk, nyaff, gowk, bozo, schlepper, goof, galoot, dumbass, dip, spud, palooka, yo-yo, drongo, alec, nong, poon, mompara, noddy, and spoony.