What do the words "canker blossom" and "waterfly" mean?
William Shakespeare is well known for his extensive use of metaphors, similes, symbolism and imagery in his writing. Although understanding his works requires knowledge of the English terms used at the time, it also requires reading the terms in their context.
"Canker blossom" is mentioned in Act 3 Scene 2 of A Midsummer Night's Dream in this context:
HERMIA (to HELENA):
You juggler! You canker-blossom!
You thief of love! What, have you come by nightAnd stol'n my love’s heart from him?
Peace.—Who comes here?
Your lordship is right welcome back to Denmark.
I humbly thank you, sir.(aside to HORATIO) Dost know this water-fly?
(aside to HAMLET) No, my good lord.
(aside to HORATIO) Thy state is the more gracious, for ’tis a vice to know him. He hath much land, and fertile. Let a beast be lord of beasts and his crib shall stand at the king’s mess. 'Tis a chough, but, as I say, spacious in the possession of dirt.
A canker blossom (or canker rose) had two meanings in Shakespeare's time: there was a dog rose, a common wild rose that was used for grafting (implying that a person was common, unimportant, and easily used by others rather than having his or her own worth), and it was also a term used to refer to an infectious skin disease on dog's ears and other soft tissues. A "canker", in general (a word often used by Shakespeare) was any infection or disease on a larger plant, causing its corruption. In general, the word "canker" can be taken to mean corruption or decay in Shakespeare.
A waterfly was another word for a dragonfly, so if applied to a person it would imply flightiness and impermanence.
Source: Meriam-Webster Unabridged Dictionary http://unabridged.merriam-webster.com/adv-unabridged.htm