Say wallahi comes from the Arabic phrase wallahi, meaning I swear by God, which entered youth slang in cities like London and Toronto as a challenge meaning prove it or are you serious. Online the phrase became a shorthand reaction in memes, voice notes and short videos whenever someone made a dubious claim. This version keeps that street slang energy in a single repeatable line, which is why it spread as a soundboard clip for arguments, banter and disbelief rather than as a formal religious expression.