奋笔疾书

fèn bǐ jí shū

to write at a tremendous speed

fèn

to exert oneself (bound form)

writing brush; writing implement (pen, pencil etc) (CL:支[zhi1],枝[zhi1])

(bound form) disease; ailment

shū

abbr. for 書經|书经[Shu1 jing1]