< Italian, plural of calamaro, calamaio, (for pesce calamaio) < Late Latincalamārium inkhorn, pen case (referring to the ink ejected by the squid), Latincalamārius pertaining to a pen; see calamus, -ary
1560s, from It. calamari, from L. calamarius, lit. "pertaining to a pen," from calamus "a writing pen," lit. "reed." So called from the cuttlefish's pen-like internal shell and perhaps also from its being full of ink.