The beast referred to on page 89 of Laurie Halse Anderson's Chains is George Washington. George Washington, or General Washington, is part of the revolution. As a member of the party who desires separation from England, Loyalists (those loyal to England) despised those who desired separation from England.
The quote refers to the beast growing too large. This means that Washington has grown too powerful. In order to "tame" his power, Washington needs to be disposed of. Essentially, he needs to be assassinated. As Loyalists, the mayor and the Locktons, do not wish to separate from England. In order to try to keep English power in the new world, Washington needs to be stopped. That said, since not all of the people around are Loyalists, the Loyalists need to use a sort of code to insure that non-Loyalists do not come to find out their plans.