Middle English sapi, of a tree or of wood, "full of sap," from Late Old English sæpig, from sæp "sap of a plant" (see sap (n.1)). The colloquial figurative sense, in reference to persons, etc., "foolish, foolishly sentimental" (1660s) might have developed from an intermediate sense of "too wet, sodden, soggy" (late 15c.), or it might have come from sappy as "containing sapwood" (mid-15c.); compare sap (n.2). Or it might be from the notion of "green, juvenile," like a sapling tree. Earlier, now obsolete, figurative senses were "full of vitality" (1550s) and "immature" (1620s). Related: Sappily; sappiness.
updated on December 22, 2021