To reach a climax or culminating point. The analogy is to an ulcer or boil that has ripened to the point of suppuration, that is, bursting. Indeed, such sores were said to “come to a head” as early as the early seventeenth century. By then the term had long since been transferred to other matters (the OED lists the earliest figurative use of it from 1340). In 1596 Edmund Spenser, describing the state of Ireland, wrote, “to keep them [i.e., these affairs] from growing to such a head.”