This is the 2879th most frequent Japanese word.
てっきり
Certainly (used when mistakenly assuming something).
The word 'てっきり' is used to express strong expectation or certainty about something, which turned out to be incorrect.
てっきり彼が来ると思っていましたが、来ませんでした。
I was certain he would come, but he didn't.
'てっきり' is employed here to denote a strong belief or assumption that turned out to be false.
彼がてっきり知っていると思っていましたが、知らなかったんですね。
I assumed he surely knew, but he didn't.
In this sentence, 'てっきり' highlights an incorrect assumption or expectation that was confidently held.