“I’m on a whiskey diet. I’ve lost three days already!” is a joke that has been credited to British prop comedian and magician Tommy Cooper (1921-1984). “I’m on a whiskey diet. I’ve lost three days already” was credited to Cooper in print by at least 2001.
   
“I’ve been on a diet for two week and all I’ve lost is fourteen days” is a similar joke that was told by comedian Joe E. Lewis in 1957.
   
     
Wikipedia: Tommy Cooper
Thomas Frederick “Tommy” Cooper (19 March 1921 – 15 April 1984) was a British prop comedian and magician.
 
Cooper was a member of the Magic Circle, and respected by traditional magicians. He was famed for his red fez, and his appearance was large and lumbering, at 6 feet 4 inches (1.93 m) and more than 15 stone (210 lb; 95 kg) in weight.
   
Google Groups: uk.gay-lesbian-bi
I’m sick of it!
Lyn David Thomas
10/25/00
(...)
Welsh poet who lived two doors from my Gran.  Claimed to a friend to be on a whiskey diet - said it was going great, he had lost two weeks already…..
 
Google Groups: alt.uk.virgin-net.oldbies
did you miss me?
Blade Runner
6/20/01
On Wed, 20 Jun 2001 07:57:50 +0100, Traci Good

wrote:
>please snd me some funnies to cheer me up!
Ok, you asked for it.  Mostly Tommy Cooper with a few of my own…
(...)
I’m on a whiskey diet. I’ve lost three days already.
 
Google Books
If Ignorance Is Bliss, Why Aren’t There More Happy People?:
Smart Quotes for Dumb Times
By John Lloyd and John Mitchinson
New York, NY: Harmony Books
2008
Pg. 339:
I’m on a whiskey diet. I’ve lost three days already!
TOMMY COOPER
 
Google Books
The Snark Handbook:
A Reference Guide to Verbal Sparring
By Lawrence Dorfman
New York, NY: Skyhorse Publishing
2009
Pg. 81:
I’m on a whiskey diet. I’ve lost three days already. —TOMMY COOPER
 
Twitter
To Make You Laugh
‏@2MakeULol
I’m on a whiskey diet. I’ve lost three days already.
~ To Make You Laugh :D
10:05 PM - 5 Jul 2014