imˈpregnablenessrare. [f.prec. + -ness.] = impregnability. 1603Knolles Hist. Turks (1621) 1170 Discouraged with the impregnablenes of the place.1647Clarendon Contempl.Ps. in Tracts (1727) 419 The strength and impregnableness of his castles and forts.