Definition of nincompoopish in US English:

nincompoopish

adjective

informal
  • Characteristic of or resembling a nincompoop; foolish, simple-minded. Also as noun (with the): nincompoops as a class.

Origin

Mid 19th century; earliest use found in The United States Democratic Review. From nincompoop + -ish.

Pronunciation

nincompoopish

/ˈnɪŋkəmpuːpɪʃ/