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