Foolish, a.
1. Marked with, or exhibiting, folly; void of understanding; weak in intellect; without judgment or discretion; silly; unwise. I am a very foolish fond old man. Shak.
2. Such as a fool would do; proceeding from weakness of mind or silliness; exhibiting a want of judgment or discretion; as, a foolish act.
3. Absurd; ridiculous; despicable; contemptible. A foolish figure he must make. Prior.
Syn. -- Absurd; shallow; shallow-brained; brainless; simple; irrational; unwise; imprudent; indiscreet; incautious; silly; ridiculous; vain; trifling; contemptible. See Absurd.
Shades of meaning
Fernald has no entry of its own for foolish; he lists it under absurd.
Where he writes about it:
Anything is irrational when clearly contrary to sound reason, foolish when contrary to practical good sense, silly when petty and contemptible in its folly, erroneous when containing error that vitiates the result, unreasonable when there seems a perverse bias or an intent to go wrong. — from absurd