By 'lump under jaw bone' the question could be referring to either the Adam's Apple, found to some extent under every persons' jaw bone, or it could be in reference to a less common lump, in which case the answer could be medical. If the first option is correct, then the answer is that the lump, usually seen in the throats of men as opposed to women or prepubescent boys, is simply what is in regular parlance called the Adam's Apple - named after the Biblical story of the original man who ate the forbidden fruit, which was said to cause the lump. It's scientifically termed a laryngeal prominence, and is a protrusion, or indeed lump, in the human neck, caused by the angle at which the thyroid cartilage surrounds the larynx. This becomes noticeable in boys going through puberty as their larynx develops and grows, and their voice box increases in size which causes their thyroid cartilage to in turn elongate and protrude. In men the thin cartilage of the of the thyroid cartilage (which are known as laminae) meet in the protrusion at an angle of ninety degrees, whilst in women they instead meet at an angle of one hundred and twenty, making women's laryngeal prominence less noticeable. Although in general women will have a smaller laryngeal prominence than men, and as a result Adam's Apple's are seen as a secondary male sexual characteristic, some women nevertheless develop masculine laryngeal prominence.
If this isn't what the question is referring to, but instead to a new lump found under the jawbone, then there are several possibilities which could explain it. However, since it could well be a symptom of a medical problem, my primary advice is to visit your doctor and explain the problem to them in greater detail, as it could be of serious importance to your health. Often lumps can become found under the jaw bone due to swelling in your lymph nodes. This in turn can be caused by several different things - though in most cases is an infection, in which case swelling of the lymph nodes is just a natural part of your body's defence as your immune system works to fight off the infection. Lymph nodes swelling can also more rarely be caused by other things, such as out of a reaction to certain drugs, glycogen storage diseases, or sarcoidosis. A worrying alternative is that swollen lymph nodes causing a lump beneath your jaw bone could be caused by a cancer of the lymphatic and blood systems. There's also the possibility that the lump is something completely different, for example a calcium spur in your neck. Because of the more dangerous possibilities of cancer, which appear similar to more harmless causes in that the lump can be painless, and the high number of possible causes, its advised that you go and see a doctor. This is especially important if the lump remains without explanation for a prolonged period, or becomes painful.
En.wikipedia.org/wiki/Laryngeal_prominence
www.patient.co.uk/health/Lymph-Glands-Swollen.htm
If this isn't what the question is referring to, but instead to a new lump found under the jawbone, then there are several possibilities which could explain it. However, since it could well be a symptom of a medical problem, my primary advice is to visit your doctor and explain the problem to them in greater detail, as it could be of serious importance to your health. Often lumps can become found under the jaw bone due to swelling in your lymph nodes. This in turn can be caused by several different things - though in most cases is an infection, in which case swelling of the lymph nodes is just a natural part of your body's defence as your immune system works to fight off the infection. Lymph nodes swelling can also more rarely be caused by other things, such as out of a reaction to certain drugs, glycogen storage diseases, or sarcoidosis. A worrying alternative is that swollen lymph nodes causing a lump beneath your jaw bone could be caused by a cancer of the lymphatic and blood systems. There's also the possibility that the lump is something completely different, for example a calcium spur in your neck. Because of the more dangerous possibilities of cancer, which appear similar to more harmless causes in that the lump can be painless, and the high number of possible causes, its advised that you go and see a doctor. This is especially important if the lump remains without explanation for a prolonged period, or becomes painful.
En.wikipedia.org/wiki/Laryngeal_prominence
www.patient.co.uk/health/Lymph-Glands-Swollen.htm
