Is it ensure, insure, or assure?
c.1300, "to perform as an acrobat," also "to fall down," perhaps from a frequentative form of Old English tumbian "dance about," of unknown origin. Related to Middle Low German tummelen "to turn, dance," Dutch tuimelen "to tumble," Old High German tumon, German taumeln "to turn, reel." Related: Tumbled; tumbling. Tumble-down (1791) originally meant "habitually falling down" and was used first of horses; sense of "in a dilapidated condition" is recorded from 1818.
1716, from tumble (v.).