What does "know one's stuff" mean?

English Language Tests, Intermediate level

ESL/EFL Test #54 [color=blue]“Lots of Stuff”, question 5

It’s quite clear she’s studied the subject very thoroughly and when she lectured, you could tell she her stuff.

(a) knew
(b) got
© showed
(d) took

English Language Tests, Intermediate level

ESL/EFL Test #54 [color=blue]“Lots of Stuff”, answer 5

It’s quite clear she’s studied the subject very thoroughly and when she lectured, you could tell she knew her stuff.

Correct answer: (a) knew

Your answer was: [color=red]incorrect
It’s quite clear she’s studied the subject very thoroughly and when she lectured, you could tell she got her stuff.
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Dear Sir

what does “know one’s stuff” mean?
why not “get” here?

thanks a lot in advance

Christina’s son

Hi Christina328,

You asked:

This means you have a very good knowledge of a subject - in other words you are an expert in that subject.

Alan