Mercutio sets Romeo up for a pun on the word "counterfeit." Romeo thinks that Mercutio is accusing him of having given them a counterfeit coin the evening before. But to give someone the counterfeit (counter-feint) can also mean to give them the slip, to duck out on them, and that’s what Mercutio is actually accusing Romeo of doing when he hopped over the wall into Juliet’s garden last night.