What is "gunnysacking"

n. The gradual accumulation of irritations and slights; bottle up.

Gunnysacking is when someone silently collects irritations and slights until "the last straw is placed on them" causing an overblown reaction.

Gunnysacking has been described as 'an alienating fight tactic in which a person saves up, or gunnysacks, grievances until the sack gets too heavy and bursts, and old hostilities pour out'. (A gunny sack is a cloth container used for carrying or storing things).