[hubwdzss] Plural of cancer

The plural of cancer is cancri.  Lung cancer and skin cancer are two common cancri.

Cancer is the Latin word for crab, which is what cut-open malignant tumors looked like to ancient Greek physicians and pathologists.  The Greek word for crab is karkinos, from which we get carcinogen.

Cancer is a second declension masculine R noun, like ager ("field") agri, from which we get agriculture.  The other more common class of second declension masculine nouns end in -us: focus foci, which demonstrates the change in the English pronunciation of the c in the opposite direction than cancer cancri.  In original Latin, c was always pronounced like k.

We also have the astronomical constellation Cancer the Crab.  The genitive case (which is the same as nominative plural for second declension masculine nouns) is used to name stars within a constellation: Beta Cancri is the brightest star in Cancer. So the word cancri is already somewhat present in English.

Also astrology: are two Cancri more likely to get along with each other than two Sagittarii?

This is even more pedantic than octopodes.