wet one's whistle

wet one's whistle (take an alchoholic drink) — промочить горло; пропустить рюмочку; выпить глоточек

Example: "The first thing I'm going to do when I get home is wet my whistle!" — " If you wet your whistle that often often you're sure to get into trouble."

------------
The idiom may derive from the fact that wetting your lips makes it easier to whistle.