The Phrase Finder, a trusted source information on the origin of expressions, says the meaning of this one is to, “Accept the inevitable impending hardship and endure the resulting pain with fortitude.” It is commonly believed that the origin was giving a soldier about to endure pain a bullet, which is malleable, to bite in order to “…concentrate their attention away from the pain and to protect against biting their own tongues.” A reference is found in Francis Grose’s 1796 A Classical Dictionary of the Vulgar Tongue: “It is a point of honour in some regiments…never to cry out or become nightingales, whilst under the discipline of cat of nine tails; to avoid which, they chew a bullet.” The Phrase Finder mentions that a common belief is that the expression came from the Sepoy native Indian fighters with the British Army who hesitated biting the greased paper cartridges of the bullets to release the gun powder. Hindus wouldn’t want to bite something that had cow fat and Muslims would resist biting something that had pig fat. Regardless, research indicates this was not the original source of the expression.