Spoiler alert! Now would be a good time to cover your eyes if you haven't finished the play yet, Shmoopsters...
Like all Shakespearean comedies, Merchant ends with the promise of one or more hook-ups. (In Shakespeare, "happily ever after" usually involves at least one wedding and/or the consummation of a marriage.) But before Portia can be reunited with Bassanio and Nerissa with Graziano, the guys have some explaining to do.
If you recall, Bassanio and Graziano gave away their rings to "Balthazar" (Portia) and his "Clerk" (Nerissa) after promising their wives they'd never part with the love tokens. (Check out "Symbols" if you need to brush up on this.) So when Portia and Nerissa whip out said rings and say they had to sleep with the lawyer and his clerk to get them, the guys think for a second that they've been "cuckold[ed]" (cheated on). Portia, however, quickly confesses that she hasn't really been sleeping around, she's just been busy cross-dressing as Balthazar and playing Judge Judy.
All is forgiven and as the cast heads inside to Portia's Belmont pad, Graziano wonders how long he'll have to wait before he can go to bed with Nerissa and then cracks a dirty joke to end the play:
Well, while I live I'll fear no other thing
So sore as keeping safe Nerissa's ring (5.1)
Graziano is making a promise of fidelity when he says he'll never again part with the ring, but there's also a sexual pun at work here, which suggests that Graziano is feeling a little anxious about whether or not he can actually guard his wife's "ring" against the sexual advances of other men. So even though things look hunky-dory for these happily wedded couples, Shakespeare allows a little uneasiness (and bawdiness) to creep in.