てっきりsurely, certainly, beyond doubt

てっきり、臆病(おくびょう)な君(きみ)は肝(きも)試(だめ)しに来(こ)ないと思(おも)ったよ！

I certainly thought that you, being timid, wouldn't come to the test of courage! {surely, certainly, beyond doubt}

彼氏(かれし)：「てっきり君(きみ)に浮気(うわき)されたと思(おも)ったが、勘違(かんちが)いだったよ。」

Boyfriend: "I surely thought you were cheating on me, but it was a misunderstanding." {surely, certainly, beyond doubt}

てっきり彼(かれ)と結婚(けっこん)するものだと思(おも)っていましたが、別(わか)れました。

I was certain that I would marry him, but we broke up. {surely, certainly, beyond doubt}

独(ひと)り言(ごと)：「てっきり今日(きょう)は晴(は)れると思(おも)ったのに…こう悪天候(あくてんこう)ではかなわない。」

Monologue: "I certainly thought it would be clear today... I can't stand this bad weather." {surely, certainly, beyond doubt}

誰(だれ)も来(こ)ないからてっきり教室(きょうしつ)の場所(ばしょ)を間違(まちが)えたかと思(おも)ったが、先生(せんせい)が教室(きょうしつ)に入(はい)ってきた。

Since nobody was coming, I certainly thought that I had made a mistake about the location of the classroom, but then the teacher entered the classroom. {surely, certainly, beyond doubt}