From rakac; a mountain ridge (as of tied summits) -- rough place.
see HEBREW rakac
H7406. rekes
[רֶ֫כֶס] noun [masculine] dubious; perhaps roughness (of ground; ? literally binding, knot); or bound up, impeded, i.e. the impassable; or mountain-chain (see Thes); — plural הָֽרְכָסִים Isaiah 40:4 ("" הֶעָקֹכ; opposed to בִּקְעָה).