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


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](http://www.weblio.jp/content/%E7%B7%9A%E3%81%8C%E7%B4%B0%E3%81%84). 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"...




> 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](http://www.dictjuggler.net/yakugo/?word=%E7%B7%9A) has this example:

[code]

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

My understanding of this collocation is:

[code]

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

The closest sense listed in Daijisen for
[線](http://dic.yahoo.co.jp/dsearch?enc=UTF-8&p=%E7%B7%9A&dtype=0&dname=0na&stype=1)
is "6. 物事を行う道筋・方針" (policy, principle), and for
[うすい](http://dic.yahoo.co.jp/dsearch?enc=UTF-8&p=%E3%81%86%E3%81%99%E3%81%84&dtype=0&dname=0na&stype=1&index=01469500&pagenum=1),
"3-5. 可能性があまりない" (slim possibility).

[code]

    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
    
[/code]

(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.




> 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](http://www.dictjuggler.net/yakugo/?word=%E7%B7%9A) has this example:

[code]

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

My understanding of this collocation is:

[code]

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

The closest sense listed in Daijisen for
[線](http://dic.yahoo.co.jp/dsearch?enc=UTF-8&p=%E7%B7%9A&dtype=0&dname=0na&stype=1)
is "6. 物事を行う道筋・方針" (policy, principle), and for
[うすい](http://dic.yahoo.co.jp/dsearch?enc=UTF-8&p=%E3%81%86%E3%81%99%E3%81%84&dtype=0&dname=0na&stype=1&index=01469500&pagenum=1),
"3-5. 可能性があまりない" (slim possibility).

[code]

    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
    
[/code]

(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.

