An act or state where a Person A does something minorly offensive to Person B, and Person B retaliates by doing something much worse in degree to Person B so that Person A will never choose to mess around again with Person B.
In other words: when someone wrongs you and you do something way worse back to make sure they don't fuck with you again.
2. That guy made fun of my brother, so I broke both his shins. Street justice, man.