1.
writing brush
A traditional brush used for writing or painting with ink, especially in East Asian calligraphy.
2.
writing, penmanship
By extension, one's skill or style in writing.
He's been busy and has become a poor correspondent.
筆不精 = reluctant to write letters
That author is known for being a fast writer and publishes several books a year.
筆が速い = to write quickly/prolifically