Answer:
The phrase "purpled thy nail" refers to the shared blood of the speaker and his beloved in the flea.
Step-by-step explanation:
"The Flea" by John Donne is a poem where a lover wants to have premarital sex and in his attempts to convince her beloved one they get bitten by a flea, which he uses as reference of their blood together as one inside of the insect and then when he receives a negative answer and he tries to kill the insect and his lover says that would be sui*cide and a murder because it has their blood in it, he told her that it doesn't have a major importance to kill the insect as it doesn't have it to love each other before marriage.