late 14c., from adjective or genitive form of Old English æspe "aspen tree, white poplar," from Proto-Germanic *aspo- (cognates: Old Norse ösp, Middle Dutch espe, Old High German aspa, German Espe), from PIE *apsa "aspen" (cognates: Lithuanian opuse). The current form in English probably arose from phrases such as aspen leaf, aspen bark (see -en(2)). 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").