Definition - having a prominent rump
Rumpy has a set of somewhat dissimilar meanings, as it may refer to certain animals (such as the Manx cat or some types of domestic fowl) who are lacking a tail where we would expect to find one, and also to animals (including people) who have more of a rump than one might expect.
In point of age, Bountiful has nearly a six years’ pull over her, and the high-bred fusion of blood in her veins has “eventuated” not exactly “a spanker,” but in a nice compact, hardy-looking cow, a little think in the jowl, and slightly inclined to be rumpy.
— The Preston Chronicle and Lancashire Advertiser (Preston, Eng.), 2 Feb. 1861
English also has steatopygic, which originated as a technical word for abnormal accumulation of adipose tissue in the gluteal region, but has broadened in use to include the simple meaning “broad-bottomed.”
It was nearly in the old “red flag” days, and those who went for a ride in this cavorting, caracoling, buck-jumping, steatopygic perambulator had some justice in their claim to be pioneers of automobilism.
— W. G. Aston, The Tatler, 26 Feb. 1930
If you want to avoid dancers’ diaphoresis and the steatopygic stance, kill two birds with one stone by getting a camouflaged camisole with the SPRINGMAID label on the bottom of your trademark.
— (advt.) Town & Country (New York, NY), Sept. 1948
For that, one would have to be a newly licensed driver again, negotiating trepidly the steatopygic gantlet of summer crabbers who bend unconcernedly over both railings, giving no quarter as they lean to dip crustaceans flippering on the tide.
— Tom Horton, The Baltimore Sun, 9 Oct. 1977