What does " Talking of which " mean in this dialog?
Harry: Yeah, Johnny’s just gone off to work...

Olivia: As always. Never stops, that boy.

Harry: Talking of which, what are you doing here? Shouldn’t you be in the shop?

Olivia: I’m just taking a break. I’m fed up of standing in that shop selling shoes all day, to tell you the truth.

Question

What does " Talking of which " mean in this dialog?
Harry:  Yeah, Johnny’s just gone off to work...

Olivia:  As always. Never stops, that boy.

Harry:  Talking of which, what are you doing here? Shouldn’t you be in the shop?

Olivia:  I’m just taking a break. I’m fed up of standing in that shop selling shoes all day, to tell you the truth.

Answer

When Harry says 'talking of which', he is referring to work. Johnny just went to work and Harry is wondering why Olivia isn't in the shop working. The expression 'talking of which' keeps the conversation on a topic that was already brought up.

Answer

"Since we are talking about this subject, here is something else related..."

Answer

I have never heard or used the phrase "Talking of which."  I have always used and heard, "Speaking of which."

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