Canker comes to us from the classical Latin ‘cancer’, meaning the sign of the zodiac, an actual crab, and anything from a whole range of tumours, abscesses, sores and even worms. According to Paulus Aeginata’s Epitomae Medicomae, the overlap between crabs and tumours in the word ‘cancer’ arises from the apparent resemblance between the swollen veins around a tumour and the limbs of a crab. This seventh-century a.d. hypothesis being the only available explanation for the link between crustaceans and carcinoma, the OED cites it.
Jumping forward to Shakespeare’s time, and the playwright’s twenty ‘cankers’ cover almost all the various meanings of the word, save that of ‘crab’. First, and most simply, a ‘canker’ is a tumour, of the kind that a misanthropic Timon wishes would “gnaw” the heart of Alcibiades. Second, a ‘canker’ is another word for the damage caused by oxidation, as is clear from the way Venus concludes a speech designed to entice Adonis into her arms: “Foul-cankering rust the hidden treasure frets, / But gold that’s put to use more gold begets.”
By far the most frequent use of canker in Shakespeare’s works, however, is in relation to its botanical meaning. A ‘canker’ is either a type of wild rose (for Hotspur, Bolingbroke is a “canker” next to the “lovely rose” that was Richard II; and Much Ado’s Don John would “rather be a canker in a hedge than a rose in [the] grace” of his brother, Prince of Arragon) or a ‘cankerworm’. This ‘cankerworm’ is a type of insect that attacks the fragile buds and flowers of plants: Blake reuses the motif in his ‘The Sick Rose’ (1794), and Shakespeare has no less than six clear references to this worm. Titania orders her fairies to hunt “cankers in the musk-rose buds” in A Midsummer Night’s Dream, Proteus and Valentine bandy talk of “cankers” in “the sweetest bud” between them at the start of The Two Gentlemen of Verona, and the famous rose-picking scene of Henry VI part I would not be complete without the future Richard III asking Somerset if his white rose “Hast not […] a canker”.
Beyond all these specific uses of the word, each more or less metaphorical or allegorical, Shakespeare makes ‘canker’ his own in two other distinctive ways, both rich with insight into the thoughts of his characters. It becomes, for example, part of a compound adjective, as in Edgar’s pithy and bitter summary of his ordeals at the end of King Lear: “Know my name is lost; / By treason’s tooth bare-gnawn and canker-bit.” In this transformation, we leave the original cankerworm far behind. Similarly, Prospero concentrates all his disdain for Caliban into the word ‘canker’, turning it into a verb in the process, to describe “A devil, a born devil, on whose nature / Nurture can never stick; on whom my pains, / Humanely taken, all, all lost, quite lost; / And as with age his uglier body grows, / So his mind cankers.”