What is meaning of "knock at"?

[Fedora]

English Language Tests, Intermediate level

ESL/EFL Test #186 "Phrasal idioms with the verb 'knock'", question 1

I usually ......... work at about 4 o'clock and run to the station to catch the train to my next job.

(a) knock on
(b) knock off
(c) knock up
(d) knock out

English Language Tests, Intermediate level

ESL/EFL Test #186 "Phrasal idioms with the verb 'knock'", answer 1

I usually knock off work at about 4 o'clock and run to the station to catch the train to my next job.

Correct answer: (b) knock off
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what is meaning of "knock at"?

Fedora

[Mister Micawber]

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It has no idiomatic meaning. 'I knocked at the door and my girlfriend opened it.'
.
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Native English teacher at Mister Micawber's