Foolish, silly, absurd, preposterous, as applied to a person, his acts, behavior, and utterances, mean ridiculous because not exhibiting good sense.
Something is foolish which does not commend itself to the judgment of others as wise or sensible or judicious.
Something is silly which seems witless, pointless, or futile.
Something is absurd which is inconsistent with accepted ideas, common sense, or sound reason; the word is applied, therefore, to ideas and projects considered impersonally as well as to persons and their acts.
Something is preposterous which is glaringly absurd or sometimes merely highly unsuitable or ridiculously out of keeping (as with a particular character or situation).