Is it ensure, insure, or assure?
mid-15c., Scottish ydy, possibly from Old Norse iða "whirlpool," from Proto-Germanic *ith- "a second time, again," which is related to the common Old English prefix ed- "again, backwards; repetition, turning" (forming such words as edðingung "reconciliation," edgift "restitution," edniwian "to renew, restore," edhwierfan "to retrace one's steps," edgeong "to become young again"). Cf. Old English edwielle "eddy, vortex, whirlpool." The prefix is cognate with Latin et, Old High German et-, Gothic iþ "and, but, however." Related: Eddies.
1810, from eddy (n.). Related: Eddied; eddying.
A current, as of water or air, moving in a direction that is different from that of the main current. Eddies generally involve circular motion; unstable patterns of eddies are often called turbulence. See also vortex.
fluid current whose flow direction differs from that of the general flow; the motion of the whole fluid is the net result of the movements of the eddies that compose it. Eddies can transfer much more energy and dissolved matter within the fluid than can molecular diffusion in nonturbulent flow because eddies actually mix together large masses of fluid. Flow composed largely of eddies is called turbulent; eddies generally become more numerous as the fluid flow velocity increases. Energy is constantly transferred from large to small eddies until it is dissipated. (See fluid mechanics.)