ἄφρων, -ον genitive -ονος

(φρήν), [in LXX for כְּסִיל, נָבָל, etc. ;]

without reason, senseless, foolish, expressing "want of mental sanity and sobriety, a reckless and inconsiderate habit of mind" (Hort; cf. MM, see word): Luk.11:40 12:20, Rom.2:20, 1Co.15:36, 2Co.11:16 12:6 12:11, 1Pe.2:15; opposite to φρόνιμος, 2Co.11:19; to συνιέντες, Eph.2:17.†

(AS)

without reason; senseless; foolish