Ji Shao Yu(纪少瑜) was a poet and writer in the Liang dynasty of the Northern and Southern Dynasties Period.
Legend has it that When he was very young, in order to be a good writer, Ji Shao Yu studied very hard, but disappointedly, he made little progress until one day he was catnapping at his desk, he dreamed that his ideal--Lu Chui (a famous writer contemporary with Ji Shao Yu) came into his room with a green-jade pen in his hand, saying, "This is a magic pen, please make good use of it."
Background And Writer Comment:
The idiom of is now used to praise someone for his/her excellent writing skill.
There is another idiom--Jiang Yan Used Up His Talents--Jiang Lang Cai Jin with similar plot with this one, but with different meaning.