c. 1300, parchemin (c. 1200 as a surname), "the skin of sheep or goats prepared for use as writing material," from Old French parchemin (11c., Old North French parcamin) and directly from Medieval Latin pergamentum, percamentum, from Late Latin pergamena "parchment," a noun use of an adjective (as in pergamena charta, attested in Pliny), from Late Greek pergamenon "of Pergamon," from Pergamon "Pergamum" (modern Bergama), the city in Mysia in Asia Minor where parchment supposedly first was adopted as a substitute for papyrus in 2c. B.C.E.
The form of the word was possibly influenced in Vulgar Latin by Latin parthica (pellis) "Parthian (leather)." The unetymological -t is an alteration in Middle English by confusion with nouns in -ment and by influence of Medieval Latin collateral form pergamentum. The technological advances that led to cheap paper restricted its use largely to formal documents, hence the sense of "a certificate" (by 1888).