Forsooth is an old adverb meaning “truly,” “indeed,” “for certain” — word for word, “for truth.” You will find it in Shakespeare and in writing that wants a medieval costume. The dictionary’s short gloss is “certainly,” and that is the old sincere use: a speaker underlining that something is so. Though actually almost nobody today says it as a straight yes. In modern English it is rare, bookish, and often a little mocking, as if the speaker were raising an eyebrow in an old cloak: “A hero, forsooth!” So the messy truth is: the meaning is “certainly / truly,” but the feeling is antique, and often sarcastic rather than solemn. It is not a word for the classroom “yes, that’s right.”

誠然、當真（古語「的確如此」；現在多半是扮古或諷刺，不是日常的「當然」）