oath (n.) Look up oath at Dictionary.com
Old English "oath, judicial swearing, solemn appeal to deity in witness of truth or a promise," from Proto-Germanic *aithaz (cognates: Old Norse eiðr, Swedish ed, Old Saxon, Old Frisian eth, Middle Dutch eet, Dutch eed, German eid, Gothic aiþs "oath"), from PIE *oi-to- "an oath" (cognates: Old Irish oeth "oath"). Common to Celtic and Germanic, possibly a loan-word from one to the other, but the history is obscure. In reference to careless invocations of divinity, from late 12c.