Is it ensure, insure, or assure?
c.1300, "mantle fastener," from Old French tassel "a fastening, clasp" (mid-12c.), from Vulgar Latin *tassellus, said to be from Latin taxillus "small die or cube," a diminutive of talus "knucklebone, ankle" (see talus (n.1)). But OED finds this doubtful and calls attention to the variant form tossel and suggests association with toss (v.). Meaning "hanging bunch of small cords" is first recorded late 14c.