What it means
てっきり expresses a firm assumption that the speaker later discovers may have been wrong. It is usually used with a following statement plus と思っていた or と思っていました, meaning that the speaker had been certain that the statement was true. The adverb often conveys surprise or disappointment when reality differs from the assumption.
Usage note
てっきり is commonly associated with a mistaken assumption, so it is not normally used for a simple present certainty that has not been challenged. It can occur in both familiar and polite sentences; the register is determined by the rest of the sentence.
How to use it
てっきり、会議は午後三時に始まると思っていました。
I was sure that the meeting would begin at three in the afternoon.
てっきり、会議は午後三時に始まると思っていた。
I was sure that the meeting would begin at three in the afternoon.
Example sentences
てっきり来月から新しい制度が始まると思っていましたが、実際には再来月からでした。
I was certain that the new system would begin next month, but in fact it was from the month after next.
てっきり会議は午後からだと思っていたが、予定表を確認すると午前中に変更されていた。
I was certain that the meeting would be in the afternoon, but when I checked the schedule, I found that it had been changed to the morning.