What is a pun in A Midsummer Night's Dream?

In Act 1 Scene 2, Bottom says he will wear a "French crown coloured beard, a perfect yellow." Quince replies with one of Shakespeare's favourite puns: "Some of your French crowns have no hair at all, and then you will play barefaced. A crown was a French gold coin; that is one meaning of "French crown". But a crown is also the top of one's head, and people who were suffering from syphilis (known at the time as "the French disease") often went bald, hence the head of someone who was suffering from the French disease was also a "French crown", a bald head resulting from syphilis. Hence the pun. Shakespeare apparently didn't mind making bald jokes even though he himself suffered from male pattern baldness and went bald at a fairly young age.