A “dead giveaway” is something that reveals a fact or an intention. A pun on the “dead giveaway” term was cited in print in 1908:
   
“A man’s last will is a dead giveaway.”
   
“New definition of a Will: a dead giveaway” was cited in print in 1946.
   
   
The Free Dictionary
dead giveaway
something that reveals a fact or an intention completely. The car in the driveway was a dead giveaway that someone was at home.
 
3 July 1908, The Call-Leader (Elwood, IN), pg. 5, col. 4:
A man’s last will is a dead giveaway.
 
28 November 1946, The Journal-Tribune and Williamsburg Shopper, pg. 6m col. 5:
New definition of a Will: a dead giveaway.
 
22 March 1948, Arkansas Democrat (Little Rock, AR), “Run of the News” by Karr Shannon, pg. 4, col. 2:
Swiped Daffynitions
Will—A dead giveaway.
 
13 September 1851, Bellingham (WA) Herald, pg. 1, col. 3:
Today’s Chuckle
Will: A dead giveaway.
Park Ridge Kiwanian
   
Google Books
January 1961, Boys’ Life, “Think and Grin,” pg. 62, col. 2:
Daffynishion: Will — A dead giveaway. — Arthur Blanchard, Holden, Mass.
   
Google Books
June 1971, Boys’ Life, “Think and Grin,” pg. 74, col. 2:
Daffynishion: Will — A dead giveaway. — Ronald Wood, Missoula, Mont.
 
OCLC WorldCat record
Wills, a dead giveaway
Author: Millie Considine; Ruth Pool; Rouben Mamoulian Collection (Library of Congress)
Publisher: Garden City, N.Y. : Doubleday, 1974.
Edition/Format:   Print book : English : 1st ed
 
Google Books
The Complete Book of Zingers
By Croft M. Pentz
Wheaton, IL: Tyndale House Publishers
1990
Pg. 53:
A will is a dead giveaway.
 
Google Books
Jokes 101
By Alex Gall
Bloomington, IN: Balboa Press
2016
Pg. ?:
A will is a dead giveaway.
   
reddit
What’s the definition of a will?
submitted August 15, 2016 by The_MartianManhunter
It’s a dead giveaway!