bird in (one's) bosom

obsolete An oath of faith, servitude, piety, or conscience that is kept guarded in one's heart, mind, and spirit. Truly, my lord, I am at your service and loyal to your name, and I shall guard the bird in my bosom till my last breath. No, Elizabeth would never betray me like that—she has always protected the bird in her bosom. Anyone involved with the king must guard the bird in their bosom. If they fail to, and the king is not in a forgiving mood, they are liable to be killed.