1580s, the name of a legal political method among the ancient Athenians by which men deemed dangerous to the liberties of the people or embarrassing to the state were banished for 10 years by public vote, from Middle French ostracisme (16c.), Modern Latin ostracismus, or directly from Greek ostrakismos, from ostrakizein "to ostracize," from ostrakon "tile, potsherd," from PIE *ost-r-, from root *ost- "bone," which also is the source of Greek osteon "bone," ostreion "oyster," and German Estrich "pavement" (which is from Medieval Latin astracus "pavement," ultimately from Greek ostrakon).
So called because the citizens each indicated the name of the man they wished banished by scratching it on a potsherd or tile. A similar practice in ancient Syracuse (with banishment for five years) was by writing names on olive leaves, and thus was called petalismos. In English, the word in the general sense of "expulsion, exclusion" (from society, etc.) is by early 17c.