In the last few days I’ve seen both “whet your whistle” and “wet your appetite,” and neither is correct. Most people’s lips don’t need to be any sharper, and appetites aren’t aroused by giving them a good soaking. Here’s how to keep wet and whet in their proper places.
This phrase has been around since the fourteenth century. There’s no hidden meaning in wet—it simply means to make wet and comes from the Old English wæt “moist, liquid.” Whistle is a little harder to decipher. It may refer to a person’s lips or throat (in the same way that “pipe” refers to the throat). The story of pub regulars using whistles to order more drinks is, according to Gary Martin of The Phrase Finder, “complete tosh.” To wet your whistle is to take a drink, to quench your thirst.
To whet your appetite is almost the opposite. Whereas wetting your whistle quenches your thirst, whetting your appetite arouses, heightens, or sharpens it. Whet means to sharpen or make more acute and comes from the Old English adjective hwæt “brave, bold” (from the Old Saxon hwat “sharp”). Blades are whetted [sharpened] by whetstones. Appetites are whetted [sharpened] by tasty morsels or glimpses of interesting or desirable things.