Pouring a shot of tequila down an ass crack (using the crack as a funnel to shoot tequila down your throat i.e. tequila luge) while tongue punching the fart box.
How far did you get with Emily last night? Nothing but a Mexican wet whistle, did about 4 and called it a night...
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.