forbidden fruit noun 1.: an immoral pleasure (as illicit sexual intercourse) 2. a.: any of several varieties of citrus fruit; especially: a small shaddock b.: the fruit of a Ceylonese tree (Tabernaemontana dichotoma) that has powerfully narcotic seeds 3.[from Forbidden Fruit, a trademark]: an American liqueur of orange color made from grape brandy flavored with shaddock