Yours, Etc.: Origins and Uses of 8 Sign-Offs
early 15c., "a calling up or driving out of evil spirits," from Late Latin exorcismus, from Greek exorkismos, from exorkizein "exorcize, bind by oath," from ex "out of" (see ex-) + horkizein "cause to swear," from horkos "oath." Earlier in the same sense was exorcization (late 14c.).