1 senseless, incautious, imprudent, impolitic, indiscreet, unwise, injudicious, ill-considered, ill-advised, misguided, short-sighted, impetuous, headlong, rash, brash, reckless, hasty, heedless, unwary, foolhardy, thoughtless, mindless:
Standing under a tree in a thunderstorm is foolish.
2 nonsensical, stupid, asinine, inane, silly, fatuous, fatuitous, dim-witted, scatterbrained, hare-brained, crazy, mad, insane, demented, irrational, erratic, unstable, crack-brained, feather-brained, bird-brained, simple-minded, light-headed, muddle-headed, numskulled or numbskulled, addle-pated, rattle-brained, bemused, confused, feeble-minded, moronic, idiotic, imbecilic, halfwitted, dull-witted, slow-witted, witless, brainless, empty-headed, blockheaded, boneheaded, thickheaded, obtuse, stolid, Colloq dumb, balmy or Brit barmy, loony, nuts, nutty, batty, dopey or dopy, soft (in the head), dim, thick, dippy, dotty, dizzy, Brit potty, daft, Slang cuckoo, goofy, screwy, wacky:
Tracy was foolish to believe him when he said he loved her.
3 preposterous, ridiculous, absurd, irrational, illogical, unreasonable, ludicrous, wild:
They have some foolish idea about building a hotel in Antarctica.