A plague on both your houses
Hold, Tybalt! Good Mercutio!
[Tybalt under Romeo's arm thrusts Mercutio in. Away Tybalt]
Mercutio:Romeo And Juliet Act 3, scene 1, 90–92
I am hurt.
A plague a' both your houses! I am sped.
Is he gone and hath nothing?
Mercutio's famous line might not be exactly the one Shakespeare wrote: instead of "a' both your houses," various old editions have "on your houses," "a' both the houses," "of both the houses," and "a' both houses." The line as I've given it here is merely editorial reconstruction—in other words, a good guess at what the "original" might have looked like, if there was only one original. This whole passage is muddled in the early texts, and in this it is not unique; what you read on the page of a modern edition of Shakespeare, let alone what you see at the theater, may not be what Shakespeare himself wrote. You're brushing up not only your Shakespeare, but also Shakespeare's editors.
In this confusing scene, Juliet's cousin Tybalt, peeved that Romeo had crashed a Capulet family ball, comes with sword drawn looking for the young lover and his cohorts. Romeo (now married to Juliet) at first refuses to be provoked by Tybalt, which enrages Romeo's mercurial friend Mercutio. Mercutio draws, Romeo intercedes, and Tybalt stabs Mercutio under Romeo's armpit. Mercutio, chagrined and disgusted, cries "a plague a' both your houses"—the feuding houses of Capulet and Montague—and complains that Tybalt has escaped unscathed. Shortly, after Mercutio has died and Tybalt has returned, Romeo, provoked once more, pays back the deed, kills Tybalt, and is therefore forced to flee Verona.