Gunnysacking is a metaphor, used in conflict resolution, which involves the act of "storing up" grievances acquired in the course of a relationship, rather than resolve them when they first occurred. The word is based on the idea of using a gunny sack (burlap bag) storing a number of items.
A typical example might occur in a marriage facing occasional difficulty. Partners may have small grievances or annoyances, which by themselves are not critical. However, over time one partner gets less happy and more annoyed. Then some incident occurs, and a full-blown argument follows. By analogy, this is the gunny sack bursting, with an 'overblown reaction'.
The term may also refer to bringing up past grievances when trying to resolve some present problem in a relationship. In which case, gunnysacking (as a tactic) has the dual effect of raising additional problems (in what is typically already a volatile, difficult situation) and presents a side-track obstacle to dealing with the current issues. Additionally, once the practice becomes a familiar pattern used by one person in a relationship, other persons may avoid reporting new issues or problems for the sake of avoiding a repeat of the gunnysacking behavior.