Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
SLOPER
As described in historical dictionaries
Patronymica Britannica (1860)
SLOPER. A slop is a kind of cloak or mantle, also a buskin or boot much used in the XV. cent. — hence Sloper. Eng. Surn. i. 112.