European tree of the poplar family  late 14c  from adjectival or genitive form of Old English spe "aspen tree  white poplar " from Proto-Germanic *aspo  adjective aspin- (source also of Old Norse sp  Middle Dutch espe  Old High German aspa  German Espe)  from PIE *aps- "aspen" (source also of Lithuanian epu  Latvian apsa  Old Prussian abse  Russian osina)  perhaps a northern European substratum word