Where did eat your words come from?

An 18 year old girl named Emily Geiger of South Carolina in 1781 volunteered to deliver a message from General Greene to General Sumter but was caught by British scouts and locked in a room until a tory women could be summoned to search her and while waiting and having memorized the message, ate the message. Upon release Emily found her way to General Sumter and spoke the message resulting in a successful attack on Rawdon.