The Collaborative International Dictionary of English v.0.48:

Sloe \Sloe\ (sl[=o]), n. [OE. slo, AS. sl[=a]; akin to D. slee,
   G. schlehe, OHG. sl[=e]ha, Dan. slaaen, Sw. sl[*a]n, perhaps
   originally, that which blunts the teeth, or sets them on edge
   (cf. Slow); cf. Lith. slywa a plum, Russ. sliva.] (Bot.)
   A small, bitter, wild European plum, the fruit of the
   blackthorn (Prunus spinosa); also, the tree itself.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sloe
    n 1: wild plum of northeastern United States having dark purple
         fruits with yellow flesh [syn: Allegheny plum, Alleghany
         plum, sloe, Prunus alleghaniensis]
    2: a thorny Eurasian bush with plumlike fruits [syn:
       blackthorn, sloe, Prunus spinosa]
    3: small sour dark purple fruit of especially the Allegheny plum
       bush

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "sloe":
   atramentous, black, black as coal, black as ebony, black as ink,
   black as midnight, black as night, coal-black, coaly, dark,
   dark as night, dark as pitch, deep black, ebony, ink-black, inky,
   jetty, midnight, night-black, night-dark, nigrous, pitch-black,
   pitch-dark, pitchy, raven, raven-black, sable, sloe-black,
   sloe-colored, tar-black, tarry