To make a gentle murmuring or hissing sound, under the action of heat, as liquids when beginning to boil; hence, to become heated gradually: said especially of liquids which are to be kept, while heating, just below the boiling-point.Figuratively, to be on the point of boiling or breaking forth, as suppressed anger.To cause to simmer; heat gradually: said especially of liquids kept just below the boiling-point.n. A gentle, gradual, uniform heating: said especially of liquids.n. A Scotch form of summer.