Cancerous comes from cancer + -ous. Cancer is from Latin cancer ('crab, malignant tumor'), so named because the swollen veins around a tumor resembled crab legs; akin to Greek karkinos. The historical trail runs through cancer in Latin. Morphologically, cancerous is transparent: it draws on cancer + -ous. Used as an adjective, cancerous now means "relating to or affected by cancer; malignantly spreading", so the present definition can be read against the older forms and meanings. The same lexical family is also visible in derived forms such as cancerously and related forms such as cancer, carcinoma, and carcinogen. The result is a borrowed or transmitted form whose modern spelling carries a compact record of its route into English.