Is it ensure, insure, or assure?
Old English cweorn "hand-mill, mill," from PIE *gwere-na- "millstone" (cf. Old Norse kvern, Old Frisian quern, Old High German quirn, Gothic quirnus; Sanskrit grava "crushing stone;" Lithuanian girna "millstone," girnos "hand mills;" Old Church Slavonic zrunuvi "mills;" Welsh brevan "hand mill"), suffixed form of root *gwere- "heavy" (see grave (adj.)).
ancient device for grinding grain. The saddle quern, consisting simply of a flat stone bed and a rounded stone to be operated manually against it, dates from Neolithic times (before 5600 BC). The true quern, a heavy device worked by slave or animal power, appeared by Roman times. Cato the Elder describes a 2nd-century-BC rotary quern consisting of a concave lower stone and a convex upper, turned by a pair of asses. Many such large querns were found in the ruins of Pompeii. The upper stone was set on a spindle that fitted into the lower. The ground grain passed down through holes in the lower stone