A plague on both your houses
Hold, Tybalt! Good Mercutio!
[Tybalt under Romeo's arm thrusts Mercutio in. Away
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.