Yours, Etc.: Origins and Uses of 8 Sign-Offs
1640s, "melting snow, snow and water," perhaps from a Scandinavian source (cf. Norwegian and Swedish slask "slushy ground;" obsolete Danish slus "sleet"), all probably imitative of the sound of sloshing. Slush fund is first attested 1839, from an earlier sense of slush "refuse fat" (1756); the money from the sale of a ship's slush was distributed among the officers, which was the original sense of the phrase. The extended meaning "money collected for bribes and to buy influence" is first recorded 1874, no doubt with suggestions of "greasing" palms.