てっきり

adverb

surely; certainly; without a doubt

Used when the speaker was convinced of something that later turned out to be wrong. Often followed by expressions of surprise or disappointment, such as 〜と思った or 〜のに.

• てっきり彼かれが来くると思おもっていたのに、来こなかった。

  I was sure he would come, but he didn't.