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.
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).