What is "to wet one's whistle"

Wet \Wet\, v. t. [imp. & p. p. Wet (rarely Wetted); p. pr. & vb. n. Wetting.] [AS. w[=ae]tan.] To fill or moisten with water or other liquid; to sprinkle; to cause to have water or other fluid adherent to the surface; to dip or soak in a liquid; as, to wet a sponge; to wet the hands; to wet cloth. ``[The scene] did draw tears from me and wetted my paper.''
--Burke.

Ye mists and exhalations, that now rise . . . Whether to deck with clouds the uncolored sky, Or wet the thirsty earth with falling showers.
--Milton.

To wet one's whistle, to moisten one's throat; to drink a dram of liquor. [Colloq.]

Let us drink the other cup to wet our whistles.
--Walton.