The Collaborative International Dictionary of English v.0.48:

Aslope \A*slope"\, adv. & a. [Pref. a- + slope.]
   Slopingly; aslant; declining from an upright direction;
   sloping. "Set them not upright, but aslope." --Bacon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aslope
    adj 1: having an oblique or slanted direction [syn: aslant,
           aslope, diagonal, slanted, slanting, sloped,
           sloping]