Accusative case of a derivative of the base of G3145 (through the idea of tentative {manipulation} that {is} unsuccessful {search} or else of punishment); {folly} that {is} (adverbially) to nopurpose: - in vain.
Verse
Matrix
Rom 1:21Because that, when they knew God, they glorified him not as God, neither were thankful; but became vainG3154 in their imaginations, and their foolish heart was darkened.