"of or pertaining to the implacental mammals," who usually are provided with a pouch for their young, 1690s, with -al (1) + Modern Latin marsupialis "having a pouch," coined from Late Latin marsupium "pouch, purse" (Classical Latin marsuppium), from Greek marsipion, diminutive of marsipos,marsippos "bag, pouch, purse," a word of foreign or Pre-Greek origin. As a noun, "a marsupial animal, an implacental didelphian mammal," from 1805.