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to close up; to shut up (e.g. behind doors)
to cut off (e.g. because a deadline has expired); to close off (e.g. subscription list)
冬は窓を締め切って暖房をつける。
In winter, I close the windows and turn on the heater.
会場の扉を締め切って式典が始まった。
The ceremony began after closing all the doors to the venue.
募集を締め切ったので応募できません。
Applications are closed, so you cannot apply.