Psycho Boyfriend Magnet wrote:
"In sooth I know not why I am so sad"
--The Merchant of Venice.
(Do you think "sooth" was slang for "truth"?)
According to the OED, the phrase "in sooth" means "truly." Sooth is an archaic word for truth and its origins are Germanic. I don’t think it was slang, but I do believe it was part of the lexicon of the day.