## The thin line? What is 線がうすい's meaning and etymology?

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I just found this phrase in my book (I don't know how much context is relevant, this is the entire sentence):

それにな、わたしが商人の線がうすい、といった理由はもうひとつある。

I looked up the phrase and found this 線が細い. It means "feeble, sensitive, fragile", and I assume that using うすい changes little in the meaning. What I gather is that the わたし is talking about the 商人's fragility. Question 1: Are these two things about right?

Question 2: Is there a clear etymology here? What exactly is someone's 線? It doesn't seem to be possible to say 線が厚い and have it mean "robust"...

1The counterpart is 線が太い. – Dono – 2012-09-10T04:51:20.320

My guess is that it might be related to profitability/business model but the best clues I have found are : その商売は利が薄かった｜The business yielded only small profits: 日本の工業は欧米の線を越えた｜Japanese industry has surpassed the level of Western countries. – Tim – 2012-09-10T06:38:58.643

Without further context, it is hard to tell, but I think that 線 here means “possibility” and that 線が薄い means “unlikely.” (To say what exactly is unlikely in your sentence, I need more context.) Interestingly, I cannot find this meaning of 線 in either Daijisen or Daijirin. But this usage of 線 is commonly seen in the form …という線がある/ない.

– Tsuyoshi Ito – 2012-09-10T11:50:44.453

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Question 1: Are these two things about right? I assume that using うすい changes little in the meaning. What I gather is that the わたし is talking about the 商人's fragility.

I can't be completely sure without more context, but the first assumption is incorrect, and therefore the second point too.

This particular usage is commonly seen in mystery stories or in a similar setting. dictjuggler's translation corpus has this example:

共犯者の線も濃厚だ: evidence of an accomplice is definitely there


My understanding of this collocation is:

線 line of reasoning
が is
うすい/濃い weak/strong


The closest sense listed in Daijisen for is "6. 物事を行う道筋・方針" (policy, principle), and for うすい, "3-5. 可能性があまりない" (slim possibility).

それにな、わたしが商人の線がうすい、といった理由はもうひとつある。
And also, there's another reason why I said it's unlikely that the merchant is the culprit.

[culprit] ------> [merchant]
うすい


(I'm wildly guessing here that there's some kind of crime involved.)

Question 2: Is there a clear etymology here?

I'm not sure how or why this usage came to be. You could get away with exchanging 線がうすい for 線がほそい behind the reader's back, and your intention will eventually get through, but they'll either do a double take, or just unconsciously replace it with the more natural collocation.

Thanks! The situation was very close to that...the narrator thought that the merchant might be able to give him a clue, but it turns out he didn't. – silvermaple – 2012-09-13T00:45:15.717