To consume a beverage of some sort.
Technically it should only be used in reference to drinking beer because of the history of the phrase:
Back in the middle ages when beer was sold in ceramic cups they would be fitted with a small whistle which could be blown upon finishing ones beverage in order to alert the bar staff that you require another beer. Therefore to get a beer would literally be "wetting ones whistle"
Martin - oh look, I finished my lovely pint of ale. Best wet my whistle with another Broadside or London Pride.
Alexi - Get me some chillie nuts or prawn cocktail crisps while your there, you slag.
by MBAP88 September 20, 2008
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To consume a beverage of some sort to cleanse one's pallet.
water is a common choice or crunk juice.

Pronounced {whet-mi-wiss-el}
Brad: Oh boy, am I thirsty! I need something to Wet my whistle!

Carson: Want some water?

Brad: I sure do!
by Cselby January 26, 2007
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Basically, it means to do something you enjoy. Technically, it means you do something to moisturize your mouth, like drink something.
I'm going to drink a beer to wet my whistle.
I like that car, it really wets my whistle.
by DP2 September 1, 2006
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