What does the 십 in 십상 mean?

 ANSWER: ten

The Korean word 십상 means "just right" or "perfect," according to Dong-A's Prime Korean-English dictionary. But 십상 comes from 십성 (十成), which literally means "a ten (十) achievement (成)." On a scale of one-to-ten, a "ten achievement" would be "a perfect score" or "the highest ranking."

Over time 십성 came to be pronounced 십상.

So, the Korean sentence 하이킹 날씨로는 십상이다 can be translated as "This is the perfect (십상이다) weather for hiking (하이킹 날씨로는)."

십성 (十成) was used to refer to the purity of such metals as "gold" (금 金) and "silver" (은 銀), so 십성금 (十成金) could be translated as "pure gold" and 십성은 (十成銀) as "pure silver."

Since ten-out-of-ten was the highest ranking, one-out-of-ten would be the lowest ranking, so since 일 (一) is the Sino-Korean word for "one," the lowest quality of gold and silver would be 일성금 (一成金) and 일성은 (一成銀), respectively.

Dong-A's Prime Korean-English Dictionary (1998)