“Squid” is thought to be a sailors’ variant of “squirt,” referring to the ink ejected by the cephalopods. Commonly used as the name of the fried culinary dish, “calamari” is also the name of a specific genus of squid and comes from the Latin calamarius, literally “pertaining to a pen.”
The Latin predecessor calamarius, is from calamus, meaning “a writing pen,” and literally “reed.”
When used to refer to an entire animal, the word calamari most often refers to the genus Sepioteuthis, commonly known as reef squids or oval squids, but once prepared as the common fried dish, many species of cephalopod can be referred to as calamari.
Cephalopod ink is primarily composed of melanin. Octopuses generally produce black ink, while squid produce bluish-black ink, and cuttlefish produce brownish ink.
Formerly, from the 1560s, “calamari” was also used as a common name for cuttlefish due to their pen-shaped internal shell and their ink.
Learn more in Once Upon a Word: An Etymology Dictionary for Kids (Rockridge Press 2020).
Featured Image Credit: Betty Wills [CC BY-SA]
Useless Etymology 
In Swahili ‘kalamu’ or ‘calamu’ means pencil (or pen) so there’s a link to Latin, probably via Arabic.
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