oath 
/ˈoʊθ/ noun
plural oaths /ˈoʊθ/ noun /ˈoʊðz/ [count] 1 : a formal and serious promise to tell the truth or to do something
▪ They were required to take/swear an oath of loyalty. [=promise formally to remain loyal] ▪ an oath to defend the nation ▪ an oath of office [=an official promise by a person who has been elected to a public office to fulfill the duties of the office according to the law] 2 old-fashioned : an offensive or rude word that is used to express anger, frustration, surprise, etc.
▪ He uttered an oath and walked away.
▪ They were required to take/swear an oath of loyalty. [=promise formally to remain loyal] ▪ an oath to defend the nation ▪ an oath of office [=an official promise by a person who has been elected to a public office to fulfill the duties of the office according to the law]
▪ He uttered an oath and walked away.
under oath also on oath
law : having made a formal promise to tell the truth in a court of law
▪ In a U.S. court of law, a witness must swear under oath to tell “the truth, the whole truth, and nothing but the truth.” ▪ He said in testimony given under oath that he was not there the night of the crime.
▪ In a U.S. court of law, a witness must swear under oath to tell “the truth, the whole truth, and nothing but the truth.” ▪ He said in testimony given under oath that he was not there the night of the crime.
