WordWeb Online

1. Broadly or extravagantly humorous; resembling farce
   "ludicrous green hair";
   - farcical, ridiculous, ridic [informal]
    
2. So unreasonable as to invite derision
   "it is ludicrous to call a cottage a mansion";
   - absurd, cockeyed [informal], derisory, idiotic, laughable, nonsensical, preposterous, ridiculous, ridic [informal]

See also: daft [Brit, informal], dumb, foolish, humorous, humourous [Brit, Cdn, non-standard], imbecile, imbecilic, stupid

Encyclopedia: Ludicrous