This adjective characterizes instability and lack of constancy. James applies it to the double-minded man in 1:8, whose wavering between God and world renders him unstable in all his ways. In 3:8, the tongue is called ἀκατάστατον κακόν—a restless evil, full of deadly poison. The classical background includes unstable situations, irregular fevers, and fickle persons. James draws on this semantic range to describe both personal instability (vacillating faith) and inherent unruliness (the untameable tongue).