Is it ensure, insure, or assure?
early 14c., "to knock down," from over- + throw (v.). Figurative sense of "to cast down from power, defeat" is attested from late 14c. Related: Overthrown; overthrowing. Earlier in same senses was overwerpen "to overturn (something), overthrow; destroy," from Old English oferweorpan (see warp (v.)).