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.
I was sure he would come, but he didn't.
てっきり今日は休みだと思って、寝坊してしまった。
I was certain today was a day off, so I overslept.
I was sure he was on a business trip, but actually he was on vacation.
I was certain it would rain, but it turned out to be sunny.