To ride to Rumford to have one’s backside new bottomed: i.e. to have a pair of new leather breeches. Rumford was formerly a famous place for leather breeches. A like saying is current in Norfolk and Suffolk, of Bungay, and for the same reason.—Rumford lion; a calf. See Essex Lion.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Rumbumtious * RumpNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes