oath

1 ENTRIES FOUND:
oath /ˈoʊθ/ noun
plural oaths /ˈoʊðz/
oath
/ˈoʊθ/
noun
plural oaths /ˈoʊðz/
Britannica Dictionary definition of OATH
[count]
1 
: a formal and serious promise to tell the truth or to do something
2 
old-fashioned : an offensive or rude word that is used to express anger, frustration, surprise, etc.

under oath

law also on oath
: having made a formal promise to tell the truth in a court of law