the.com/bone to pick

means A complaint or point of contention you intend to raise with someone, often one you've been holding onto before the chance arrives.

from Recorded in English since the 16th century, the phrase pictures a dog gnawing at a bone — an absorbing, time-consuming task that takes patience and isn't easily abandoned. Having 'a bone to pick' with someone casts the grievance itself as that bone: something you've been saving to chew over, and won't put down until it's settled.