Anything cockamamie is ridiculous and far-fetched. A cockamamie plan is not likely to work.
This is a silly-sounding word for things that are silly, ludicrous, foolish, and impractical. Faking your own kidnapping to get out of a math test is a cockamamie plan. Wearing a fake mustache everywhere for no reason is cockamamie. Lots of comedies involve cockamamie plans and schemes, where the characters are trying to do something bizarre and unlikely to work. A cockamamie idea is the opposite of a practical, logical idea.