Old English Pentecosten "Christian festival on seventh Sunday after Easter," from Late Latin pentecoste, from Greek pentekostē (hēmera) "fiftieth (day)," fem. of pentekostos, from pentekonta "fifty," from pente "five" (from PIE root *penkwe- "five"). The Hellenic name for the Old Testament Feast of Weeks, a Jewish harvest festival observed on 50th day of the Omer (see Leviticus xxiii.16).
"of or pertaining to the ancient people of southern Italy," 1590s, from Latin Osci, Opsci (plural) "Oscans," literally "worshippers of Ops," a harvest goddess, the name related to Latin ops (genitive opis) "abundance, plenty, wealth, riches," from PIE root *op- "to work, produce in abundance." As a noun, in reference to a member of the people occupying much of southern Italy in ancient times, 1753; in reference to their language, by 1813.