1An adverb expressing strong confidence that something will be, or is, the case — 'surely', 'certainly', 'most likely'.
今度こそきっとうまくいくよ。
I'm sure it'll work out this time.
彼はきっと私のことを覚えていないだろう。
He surely doesn't remember me.
頑張ればきっと合格できる。
If you work hard, you'll surely pass.
明日はきっと晴れると思う。
I think it will surely be sunny tomorrow.
2Used especially as キッと to describe a sharp, stern look or manner — such as glaring or scolding sharply.
母はキッとした顔で私をにらんだ。
My mother glared at me with a stern look.
彼はキッと表情を引き締めて答えた。
He answered with his expression sharply set.
3Describes something tightened firmly with no looseness or slack.
靴のひもをきっと結んでおいた。
I tied my shoelaces tightly.
ネジをきっと締めてから確認してください。
Please check after tightening the screw firmly.
4An archaic, literary usage meaning 'suddenly'; rarely encountered in modern conversation.
（文語）きっと立ち上がり、彼は部屋を出て行った。
(Literary) He suddenly stood up and left the room.