Is it ensure, insure, or assure?
c.1400, from Latin ergo "therefore, in consequence of," possibly from *ex rogo "from the direction," from ex "out of" (see ex-) + root of regere "to guide" (see regal).
ergo- pref. Work: ergometer.
Latin word meaning “therefore”; usually used to show a logical conclusion: “Birds are warm-blooded animals, and reptiles are cold-blooded animals; ergo, no bird is a reptile.”