Example : An idiotic idea.

Synonyms : crackbrained

Example : The absurd excuse that the dog ate his homework.
That's a cockeyed idea.
Ask a nonsensical question and get a nonsensical answer.
A contribution so small as to be laughable.
It is ludicrous to call a cottage a mansion.
A preposterous attempt to turn back the pages of history.
Her conceited assumption of universal interest in her rather dull children was ridiculous.

Synonyms : absurd, cockeyed, derisory, laughable, ludicrous, nonsensical, preposterous, ridiculous

Synonyms : imbecile, imbecilic