Have a drink, as in I'm just going to wet my whistle before I go out on the tennis court. This expression uses whistle in the sense of “mouth” and may allude to the fact that it is very hard to whistle with dry lips. [Late 1300s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.