means A solemn promise that binds you to truth or duty, often sworn before a witness or a higher power — or, loosely, a swear word flung in anger.

from From Old English āth, a word for a sworn pledge, with cousins across the Germanic languages — Old Norse eiðr, Old High German eid. The deeper roots are murky, but the word has always carried weight: to swear an oath was to stake your word against the gods or the law. The slide from 'sacred vow' to 'curse' came later, as the holy names invoked in solemn swearing got borrowed for cursing — so the same breath that once sealed a promise could also blaspheme it.