answer:Here is one person’s theory: “From the eighteenth century on, writers and especially poets have talked about duties, delays, other negative things as “weighing” upon people or their spirits. The case of the weight upon the chest is interesting, as references to body parts often have a physiological basis. Having “a lump in your throat” is an example on one which has been elucidated as a feeling which results from s specific chemical change in the body. Having something weighing on the chest almost invariably accompanies nightmares (although the converse is not necessarily true—nightmares do not always occur if something weighs on your chest). ” Source