Source : Webster's Revised Unabridged Dictionary (1913)

Sleet \Sleet\, v. i. [imp. & p. p. {Sleeted}; p. pr. & vb. n.
   {Sleeting}.]
   To snow or hail with a mixture of rain.

Sleet \Sleet\, n. (Gun.)
   The part of a mortar extending from the chamber to the
   trunnions.

Sleet \Sleet\, n. [OE. sleet; akin to MHG. sl?z, sl?ze
   hailstone, G. schlosse; of uncertain origin.]
   Hail or snow, mingled with rain, usually falling, or driven
   by the wind, in fine particles.

Source : WordNet®

sleet
     n : partially melted snow (or a mixture of rain and snow)
     v : precipitate as a mixture of rain and snow; "If the
         temperature rises above freezing, it will probably sleet"