Water under the bridge refers to past events, especially fights or disagreement, that are forgiven, forgotten, or otherwise no longer considered important.
To square up means to get into a fighting position. However, it can also mean to settle a financial balance. Or, it sometimes means to physically or metaphorically reconcile two opposing objects, ideas, or people.
The Supremacy Clause is the common moniker of Article VI, Clause 2 of the United States Constitution. The clause establishes the Constitution and federal laws as the supreme Law of the Land, above state laws.
Boo Radley is a fictional character from Harper Lees 1960 novel To Kill a Mockingbird. He is a mysterious, reclusive man and, as such, the frequent subject of childrens ghastly legends.