n. An unlined or undyed waistcoat; a single garment; -- opposed to
n. In certain games at cards, as whist, a single card of any suit held at the deal by a player;
n. [ Cf. Swingletree. ] The pivoted or swinging bar to which the traces of a harnessed horse are fixed; a whiffletree. [ 1913 Webster ]
☞ When two horses draw abreast, a singletree is fixed at each end of another crosspiece, called the doubletree. [ 1913 Webster ]