References

In his article, The Historical relationship between the concept of tumor and the ending -oma, Harry Keil, cites Hippocrates in Oeuvres completes d'Hippocrate, Celsus in De medicina, and Galen from several sources, in explaining the origin of the term cancer for a particular disease.

The Hippocratic writers (fifth to fourth century B.C.) used the words carcinos (καρκινος) and carcinoma (καρκινψμα) in practically synonymouse sense, and both meant crab. ... These words were nowhere precisely defined, and they were applied indiscriminately to intractable ulcerations as well as to lesions that seem to correspond to our modern malignant neoplasms.

The Hippocratic concept of carcinos and underwent a radical change in the hands of Celsus (first century A.D.). Translating the former into the Latin cancer (meaning crab) and transliterating the latter into the Latin as carcinoma (a word foreign to the Romans), Celsus gave these terms different interpretations. Cancer meant generally a deeply penetrating type of ulceration, whereas under carcinoma Celsus furnished a remarkable discussion of what would correspond to pre-malignant and malignant neoplactic lesions.

In his article, The Crab, the turkey and a malignant tale from the year of the rooster, Adrian Reuben quotes Pliny the Elder (from Philemon Holland's 1601 English translation of "The History of the World") as describing cancer as

a swelling or sore comming from melancholy bloud, about which the veins appeare of a blacke or swert (dark) colour, spread in the manner of Creifish clees (claws). whereupon it tooke tha name in Latine, like as in Greeke Carcinoma.

According to legend, Galen in the 2nd Century CE and/or Paulus of Aegina in the 7th Century CE thought the disease was so-called because '...it appears at length with turgid veins shooting out from it, so as to resemble the figure of a crab; or as others say, because like a crab, where once it has got, it is scarce possible to drive it away.'

Galen proves to be more specific than his predecessors. For one thing, he distinguishes between name of diseases that refer either to symptoms like 'phagedaina' (an ulceration that devours or consumes) or herpes (an ulceration that make its way like a crawling beast) and to those that are named because of their similarity to externatl objects like cancer (crab) and polyp (many footed one).