An eddy, a whirlpool; an eddying or whirling body of water, in later use also of cloud, dust, etc.
‘The water that by some interruption in its course, runs contrary to the direction of the tide or current’ (Adm. Smyth); a circular motion in water…
An eddy or counter-current.
U.S. = back-set, n. 2 (In recent dictionaries.)