Bone of contention is something that two or more people argue about strongly over a long period of time.
Main issue of a disagreement; something to quarrel about. For example, Grandfather's will was a bone of contention for the whole family. This expression alludes to two dogs fighting (contending) over a single bone.
A bone of contention is the cause of a long-standing argument. ... This expression comes from the fact that two dogs would often fight over a bone, with neither animal wanting to give in to the other. Today, anything can become a bone of contention between two or more people.
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