Final answer:
The phrase refers to a critical scene in Shakespeare's 'Romeo and Juliet' where Tybalt stabs Mercutio under Romeo's arm, escalating the feud between the Montagues and Capulets and contributing to the tragedy.
Step-by-step explanation:
The line "Tybalt under Romeo's arm thrusts Mercutio in" is from Act 3, Scene 1 of William Shakespeare's play Romeo and Juliet. This moment is a pivotal point in the tragedy, leading to a series of fatal events. Here, Tybalt, Juliet's cousin, stabs Mercutio, Romeo's friend, under Romeo's arm. This act of violence escalates the ongoing feud between the Montagues and the Capulets and propels Romeo towards revenge, which ultimately contributes to the tragic ending where both young lovers die. This scene is an example of Shakespeare's ability to intertwine fate and individual actions to advance his plots and develop his characters, a common theme also found in other works such as Macbeth and the costume renderings add to understanding the historical and theatrical context of the play.