by 1685 (n.) "a person of Pennsylvania;" by 1698 (adj.) "of or pertaining to Pennsylvania," from Pennsylvania + -an. In reference to a geological system, attested from 1891.
Entries linking to pennsylvanian
Pennsylvania
American colony, later U.S. state, 1681, literally "Penn's Woods," a hybrid formed from the surname Penn (Welsh, literally "head") + Latin sylvania (see sylvan). Not named for William Penn, the proprietor, but, on suggestion of Charles II, for Penn's late father, Admiral William Penn (1621-1670), who had lent the king the money that was repaid to the son in the form of land for a Quaker settlement in America. The story goes that the younger Penn wanted to call it New Wales, but the king's secretary, a Welshman of orthodox religion, wouldn't hear of it. Pennsylvania Dutch (adj.) in reference to the German communities of the state, which retained their customs and language, is attested from 1824.
-an
word-forming element meaning "pertaining to," from Latin -anus, adjective suffix, in some cases via French -ain, -en. From PIE *-no-.