A Bone to Pick

This phrase dates to the mid-16th century and draws its image straight from dogs. A dog given a bone will focus on it single-mindedly, gnawing and worrying at it until every scrap is gone, and two dogs given one bone between them will often fight over it. "Having a bone to pick" with someone borrows that image for a matter that needs your full, undivided attention, usually an unresolved grievance you intend to settle. A closely related expression, "bone of contention," describing the actual subject of a dispute rather than the act of raising it, comes from the same canine image and the same general period.