Is it ensure, insure, or assure?
late 14c., from adjective or genitive form of Old English æspe "aspen tree, white poplar," from Proto-Germanic *aspo (cf. Old Norse ösp, Middle Dutch espe, Old High German aspa, German Espe), from PIE *apsa "aspen" (cf. Lithuanian opuse). The current form in English probably arose from phrases such as aspen leaf, aspen bark. Its leaves have been figurative of tremulousness and quaking since at least early 15c. (an Old English name for it was cwicbeam, literally "quick-tree").