プログレッシブ和英中辞典(第4版)の解説
てっきり
てっきり雨だと思っていたら晴れた
I was sure it was going to rain, but it turned out to be a fine day.
どろぼうはてっきり逃げたと思ったら屋上に隠れていた
We were convinced that the thief had got(ten) away, but he was hiding on the roof.
てっきり電話をくださると思っていました
I took it for granted that you would phone me.