The phrase "form an aggrieved majority" refers to a group of people who are upset and feel they have been wronged by a decision or action. Some synonyms for this phrase may include "become a disgruntled majority," "unite as a dissatisfied group," or "coalesce as a frustrated bloc." Other possibilities could include "assemble as an indignant majority," "band together as a perturbed coalition," or "congregate as a discontented faction." Whatever the wording, the underlying sentiment is one of frustration and disappointment with a perceived injustice or mistreatment.