What was the unofficial motto of the sons of liberty?

The unofficial motto of the Sons of Liberty was "no taxation without representation".

Explanation:

Sons of Liberty was a political group of American patriots that originated in the Thirteen Colonies in the wake of the American War of Independence in the 1770s.

The Sons of Liberty were formed to encourage change in the British government's treatment of their colonies in the years following the end of the French and Indian War, that ended in 1763.  

Groups that identified themselves as Sons of Liberty existed in almost every British colony. The organization spread to new areas month after month and spread to more and more different colonies.

The main objective of this society was to fight against the injustices made by Britain, since it imposed numerous duties on the overseas colonies, such as the Stamp Act, concerning the press. The Sons of Liberty became famous for their sometimes vandalistic acts that damaged the British: famous in this regard was the protest to the Tea Act, where at the port of Boston (Massachusetts) some Sons of Liberty, disguised as redskins, attacked a ship laden with tea and threw the cargo out of the ship.