The word calamari was borrowed into English from 17th-century Italian, where it functioned as the plural of "calamaro" or "calamaio." The Italian word, in turn, comes from the Medieval Latin noun calamarium, meaning "ink pot or "pen case," and can be ultimately traced back to Latin calamus, meaning "reed pen." The transition from pens and ink to squid is not surprising, given the inky substance that a squid ejects and the long tapered shape of the squid's body. English speakers have also adopted "calamus" itself as a word referring to both a reed pen and to a number of plants.
1826, in the meaning defined above
Italian, plural of calamaro, calamaio, from Medieval Latin calamarium ink pot, from Latin calamus; from the inky substance the squid secretes