adj

1. having a mental age of three to seven years

Synonym: imbecileimbecilic

2. insanely irresponsible

e.g. an idiotic idea

Synonym: crackbrained

3. incongruous
nviting ridicule

e.g. 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

Synonym: absurdcockeyedderisorylaughableludicrousnonsensicalpreposterousridiculous