Checking it in
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to show your ticket at an airport so that you can be told where you will be sitting and so that your bags can be put on the aircraft:
Checking it in meaning & definition 1 of Checking it in.
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to contact someone by making a phone call, short visit, etc., usually in order to make sure there are no problems or to tell them that there are no problems:
Checking it in meaning & definition 2 of Checking it in.
