What does Mercutio mean when he says "a plague o' both your houses"?

When Mercutio utters the famous line "a plague o' both your houses" in Shakespeare's "Romeo and Juliet," he is expressing his frustration and anger at both the Capulet and Montague families. Mercutio is mortally wounded in a brawl between Tybalt (a Capulet) and Romeo (a Montague). As he lies dying, Mercutio condemns the two feuding households, blaming their ongoing conflict for his imminent death.

This statement is a curse, a wish for misfortune or calamity upon both the Capulet and Montague families. Mercutio's words highlight the tragic consequences of the ancient grudge between the two houses, and his death becomes a turning point in the play, intensifying the animosity and leading to the tragic fate of Romeo and Juliet.