|bone of contention|
|the grounds or subject of a dispute|
bone of contention
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. In slightly different guise, bone of dissension, it was used figuratively in the 16th century and took its present form in the early 1700s.