続日本紀
One is that it originated from the on'yomi (Sino-Japanese reading) of the Chinese name 昆布 (kūnbù). The kanji itself already could be seen in "Shōsōin Monjo" (8th century) and "Shoku Nihongi" (797) in Japan, and furthermore trace back in China, as early as 3rd century, to the book "Wupu Bencao" (around 239). Li Shizhen wrote the following in his "Bencao Gangmu" (1596):