Croup1, n.
(krōōp), n. [Fr. croupe, hind-quarters, croup, rump, Pr. cropa, Sp. grupa, Pg. garupa, It. groppa; Fr. groupe, a group, cluster, clump, Sp. grupo, gorupo, It. gruppo, gruppo. Cf. Crop and Icel. kryppa, hunch, hump, bending, Gael. crup, to bend, contract.] The hinder part or buttocks of certain quadrupeds, especially of a horse; hence, the place behind the saddle.
So light to the croup the fair lady he swung, So light to the saddle before her he sprung. W. Scott.