One summer night, when the moon was very bright he suddenly saw a girl descending(下降) slowly from the sky. He observed the girl closely, and found that the dress she was wearing was seamless(无缝的) . He was puzzled, and asked why. The girl answered,"Heavenly clothes are not sewn with needle and thread."
传说古代太原人郭翰在夏夜里乘凉，见一个仙女从天上下来，她身穿白衣，美丽绝伦。她告诉他她名叫织女。郭翰仔细欣赏织女的衣裳浑然一体，竟看不出一丝线缝，好奇问织女。织女答道：“天衣本非针线为也。”
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.
神话传说，仙女的衣服没有衣缝。比喻事物周密完善，找不出什么毛病。