His pen name Shiki was an alias for "Hototogisu"(lesser cuckoo), a bird which is said to chirp until it spits out blood, a metaphor for the image of himself suffering from tuberculosis and lung hemorrhage.
雅号の子規とはホトトギスの異称で、結核を病み喀血した自分自身を、血を吐くまで鳴くと言われるホトトギスに喩えたものである。