Hamlet’s wordplay here makes use of several double meanings, making his banter with Ophelia take a decidedly bawdy turn:

• He offers to “lie” in Ophelia’s “lap,” and when she refuses, he assures her he meant only to rest his head there.
• When he asks if she thought he meant “country matters,” his phrase could refer to something rural and unsophisticated, or it could allude to a derogatory term for women.
• And finally, when Ophelia says, “I think nothing,” nothing is also a sexual pun, which is why Hamlet says it's a “fair thought.”