also: いかのこう
noun
cuttlebone
The internal shell of a cuttlefish, often used as a calcium supplement for pet birds or for polishing.
インコのためにイカの甲を買った。
I bought a cuttlebone for my parakeet.
Compound of イカ (squid/cuttlefish) and 甲 (shell/armor). The exact historical derivation is straightforward.