Sap \Sap\, n. [AS. s[ae]p; akin to OHG. saf, G. saft, Icel. safi; of uncertain origin; possibly akin to L. sapere to taste, to be wise, sapa must or new wine boiled thick. Cf. {Sapid}, {Sapient}.] 1. The juice of plants of any kind, especially the ascending and descending juices or circulating fluid essential to nutrition. [1913 Webster] Note: The ascending is the crude sap, the assimilation of which takes place in the leaves, when it becomes the elaborated sap suited to the growth of the plant. [1913 Webster] 2. The sapwood, or alburnum, of a tree. [1913 Webster] 3. A simpleton; a saphead; a milksop. [Slang] [1913 Webster] {Sap ball} (Bot.), any large fungus of the genus Polyporus. See {Polyporus}. {Sap green}, a dull light green pigment prepared from the juice of the ripe berries of the {Rhamnus catharticus}, or buckthorn. It is used especially by water-color artists. {Sap rot}, the dry rot. See under {Dry}. {Sap sucker} (Zo["o]l.), any one of several species of small American woodpeckers of the genus {Sphyrapicus}, especially the yellow-bellied woodpecker ({S. varius}) of the Eastern United States. They are so named because they puncture the bark of trees and feed upon the sap. The name is loosely applied to other woodpeckers. {Sap tube} (Bot.), a vessel that conveys sap. [1913 Webster]