Uncertain; perhaps of Semitic origin, such as Hebrew פִּטְרִיָּה (pitriyá, “mushroom”), Classical Syriac ܦܛܘܪܬܐ (peṭṭorta), Arabic فُطْر (fuṭr, “mushroom”). Or, related to Old French bot (“toad”), recalling the shape of a big mushroom. Or, from Middle French poistron, from Late Latin posteriōnem, from Latin posterus (“following, next”).