Responses
That is very interesting and certainly not the "usual" turn of events.
If this is part of what the herbal infusion does, I am sure the servant was trying to make things easier for Nakyum.
Maybe it increases sensitivity because it looked like NK didn't feel that way until SH started using the brush on his chest. I think he would have fought SH more as he was pushing him down if he had been turned on prior.
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So on the latest, what I believe is being implied is that the "herbal infusion" that the servant bought secretly (the one that Yoon uses) really puts you in the mood a lot. So that's why he most likely bought it without telling Yoon and has been dosing the Painter with it.
The Painter refuses to drink anymore saying it makes him feel strange (I think kinda horny). Yoon likely doesn't know the Painter's being given this herbal fusion since the old man got it secretly.
Painter goes to paint and ends up hard. But before Yoon can finish him, Yoon' brother shows up (we don't see just hear "The Young Master is here").
Duh-duh-DUHN!