early 15c. (implied in flostrynge), "bluster, agitate," probably from a Scandinavian source (compare Icelandic flaustr "bustle," flaustra "to bustle"), from Proto-Germanic *flaustra-. Originally "to excite," especially with drink; sense of "to flurry, confuse" is from 1724. Related: Flustered; flustering; flustery. As a noun, 1710, from the verb.