From 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]