To precipitate as a mixture of rain and snow.

ETYM Old Eng. sleet; akin to Mid. High Germ. slîz, slîze hailstone, German schlosse; of uncertain origin.
Partially melted snow (or a mixture of rain and snow).
Precipitation consisting of a mixture of water and ice.
Technically, it refers to precipitation in the form of ice pellets smaller than 0.2 in/5 mm. The icy coating when rain freezes on trees and streets is also called sleet.