From 注意(ちゅうい) (chūi, “warning”) + 書(か)き (kaki, “writing, 連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 書(か)く (kaku, “to write”).”).
ちゅういがき • (chūi-gaki)