surely; I swear; upon my word
Emphatic adverbial use of the て-form of 誓う (to swear), expressing strong assertion or conviction. Often used in somewhat dramatic or old-fashioned speech.
I swear I will never tell a lie.
I swear I am innocent.
Used with a negative predicate to strongly deny something. Equivalent to 'absolutely not' or 'never'.
I would never do such a thing.
I swear this is the truth.
決して is the standard word for strong negation and pairs with negatives just like 誓って, but without the swearing nuance.
断じて shares 誓って's dramatic, emphatic tone when denying something, though it lacks the literal sense of swearing an oath.
Adverbial use of the て-form of 誓う (to swear).