PREPOSTEROUS
adjective
1. So senseless as to be laughable: absurd, foolish, harebrained, idiotic, imbecilic, insane, lunatic, mad, moronic, nonsensical, silly, softheaded, tomfool, unearthly, zany. Informal cockeyed, crazy, loony, loopy. Slang balmy2, dippy, dopey, jerky, sappy, wacky. See ability/inability, knowledge/ignorance.
2. Beyond all reason: obscene, outrageous, ridiculous, shocking, unconscionable, unreasonable. Idioms: out of bounds, out of sight. See usual/unusual.