大牙 literally means 'big tooth' and can refer to either a molar (back tooth) or a front tooth, depending on context. It appears prominently in the common idiom 笑掉大牙 (xiàodiào dàyá), meaning 'to laugh off one's back teeth' — i.e., to laugh heartily at someone. In everyday usage, 大牙 is more often heard in this idiom than in literal dental descriptions.