The shamrock (Irish seamróg) is a plant with trifoliate leaves and it is a symbol of St. Patrick's Day because tradition has it that St. Patrick used the plant to explain the theological doctrine of the Holy Trinity (three persons in one God). The word is a diminutive of Irish seamar, "clover." The ancient Druids associated the shamrock with the coming of spring and the rebirth of the natural world at the vernal equinox. The shamrock was later adopted as the national symbol of Ireland. The phrase "to drown the shamrock" means "to go drinking on St. Patrick's Day." Clover is the common name for this species of trefoil, but it was spelled various ways prior to 1600, which is also what happened to shamrock until around 1577.