For certain

наречие 1) подлинно; достоверно Синоним(ы): authentically, for sure 2) определённо, заметно Синоним(ы): distinctly, definitely

Большой англо-русский словарь

For certain

phrasal as a certainty; assuredly Example: the cause is not known for certain

Merriam-Webster's Collegiate Dictionary