What does てっきり mean?
てっきり is a Japanese adverb meaning “surely, certainly, beyond doubt” in English. It is written as てっきり in hiragana, and tekkiri in romaji. This word is part of JLPT N1 vocabulary.
Example sentence
てっきりあなたが我々といっしょに来られるものと思っていました。
I took it for granted that you would come with us.