There was a man called Guo Han in the Tang Dynasty (618-907). One summer night, when the moon was very bright, he suddenly saw a girl descending slowly from the sky. He observed the girl closely, and found that the dress she was wearing was seamless. He was puzzled, and asked why. The girl answered, ‘Heavenly clothes are not sewn with needle and thread.’
This idiom is used metaphorically to indicate the flawless handling of things. It can also be used to indicate a perfectly written poem or other literary article.