"Bitter line" Meaning

There was an american patient in a Japanese doctor's house. He was cured and treated well. Now, he was recovering. But, the Japanese doctor had been disliking the man, for he was american. On the third day after his operation, the doctor came and saw his patient sitting on bed on his own. He scolded him and told him to lie down. He did it, closed his eyes, turned his face to the wall and said

"Okay"

Then the author says:

His mouth a bitter line.

What's the meaning of bitter line? line? Web's empty.

Edit:

I got something: This shows that "line" can be thought of as "an approach", so the patient approaches the doctor in a bitter way. That is, he was hurt, in anger, he was showing hate to the doctor. Could it be ever well?

I'd argue the meaning is more literal and he's trying to create a visual for the reader. The author is describing the patient's mouth. As opposed to a smile or a smirk, an arc or a curve, it's a line. To refer back to the link you provided, by definition, 'a long, thin mark on the surface of something'. So, the author is asking you to picture his mouth in this way, so you can see his bitterness for the doctor, as opposed to just telling you he felt bitter. It's actually a nice example of show don't tell.

Think of emojis, specifically the basic round head and a line for the lips. 😑 😐😳😔. It is meant to invoke just such an image, and then call it a bitter line, instead of saying that the patient (or doctor, not sure from context which) was bitter.

Bitter is a hard emotion to describe in writing. Dancing with joy, quivering with terror -- easy. Bitter doesn't have any particular facial expressions or actions associated with it, instead bitter is attached to other expressions: bitter smile, bitter frown, or in this case bitter non-nonexpression.