To put in is to spend time or effort on something. Hours, work, or a good word for somebody.
The effort sense is the commonest, so putting in the hours and putting in a shift are both compliments about hard work. It has several other jobs: putting in an application means submitting one, putting in a good word for somebody means recommending them, and a ship putting in at a port is stopping there. The noun input comes from the same idea and belongs entirely to work and data.
From put + in.
Etymology from Wiktionary, CC BY-SA
Practised in Phrasal Verbs, A Man Called Ove
First appears in chapter 7 of A Man Called Ove.
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