truthfully
adv. manneradv. in a way that is honest and tells the real facts. You use this when you want to show you are not lying or hiding anything.
adv. in a manner that is honest and consistent with the facts. Often used as a sentence-modifying disjunct to emphasize the speaker's sincerity.
I can truthfully say that I did my best.
When the manager asked about the missing files, Sarah answered truthfully even though she was nervous.
The witness swore to answer every question truthfully, knowing that any deviation from the facts could lead to a charge of perjury.
From truthful + -ly.
Can function as a manner adverb or a sentence adverb; in the latter case, it often appears at the start of a clause.
Truthfully, I don't like itTo be honest, I don't like itWhile 'truthfully' is grammatically correct at the start of a sentence, learners often over-rely on it when 'to be honest' or 'frankly' is more natural for expressing an opinion.