This adjective means "not bending," "unswerving," or "steadfast"—describing something that doesn't incline to either side but remains firm and fixed. Classical usage applied it to things straight and perpendicular, impartial judgments, and persistent fevers. The author of Hebrews uses it in Hebrews 10:23 exhorting believers to "hold fast the confession of our hope without wavering." The term emphasizes undeviating firmness—not bending under pressure, not turning aside through doubt or persecution. The confession must be held with steady grip, neither compromised nor abandoned, remaining steadfast regardless of opposition or trial.