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歃 is composed of 臿(the picture of a pestle thrusting into a mortar)and 欠(lack, the picture of a man opening his mouth). Ancient Chinese smeared their mouth with animal's blood when taking oath, in order to show the faith. 臿 represents thrusting, if we want to get the blood from the animal, we need to stab its skin, so 歃 which is composed of 臿 and 欠, means an ancient Chinese oath.