Is this correct meaning?

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1

수나 - “embroidery”,Is this correct meaning?

2

No, it doesn’t mean embroidery here, although it is one of the many meanings of “수” (number; way; embroidery, etc).

수나 is constructed from 수 (noun) + 나 (particle), where:

  • 수 - way, means, idea, method. Ex: 좋은 수가 있다 - I have a great way. 무슨 수가 없을까? - Isn’t there some way?
  • 나 - particle similar to 가 but with a sense of “as much as”, “no less than”, similar to “-라도”.
  • 큰 수나 난 듯이 → as if (듯이) he came upon (난) a great idea (큰 수나).

Translation: 사슴의 발자국을 보고 큰 수나 난 듯이 덫을 놓아 두었습니다
--------------> Seeing the deer’s paw prints, he set a trap there as though he came upon a great idea.
--------------> Seeing the deer’s paw prints, he set a trap there thinking he came upon a great idea.