Is it ensure, insure, or assure?
early 15c. (implied in flostyrynge), from a Scandinavian source (cf. Icelandic flaustr "bustle," flaustra "to bustle"). Originally "to excite," especially with drink; sense of "to flurry, confuse" is from 1724. Related: Flustered; flustering. As a noun, 1710, from the verb.