“Capital punishment” is the death penalty. An old joke defines the income tax as a punishment for having capital, or “capital punishment.” “Capital punishment—the income tax” is from the New York City humor magazine Life on May 12, 1921.
 
Jeff Hayes has been credited with the line since the 1970s, but he (whoever Jeff Hayes is) almost certainly used a 50-year-old joke line at that time.
   
 
Hathi Trust Digital Library
12 May 1921, Life magazine, “Life Lines,” pg. 690, col. 3:
Capital punishment—the income tax.
   
Hathi Trust Digital Library
22 January 1922, Life magazine, “Life Lines,” pg. 6, col. 1:
Capital punishment—the income tax.
 
22 June 1950, Portsmouth (OH) Times, “Minego’s Sports Gossip” by Pete Minego, pg. 28, col. 4:
Pete: Wot is your definition for capital punishment? The old income tax, buddy.
 
Google Books
Peter’s Quotations:
Ideas for Our Times
By Laurence J. Peter
New York, NY: Morrow
1977
Pg. 463:
Capital punishment: The income tax.—Jeff Hayes
 
Napa Valley Register
18 February 2008, Napa Valley Register (CA), “The economic stimulus”: ‎
Notable Quote: “Capital punishment: income taxes.” Jeff Hayes
 
Google Books
As Certain As Death:
Quotations About Taxes
By Jeffrey L. Yablon
Arlington, VA: Tax Analysts
2010
Pg. 20:
Capital punishment: The income tax.
—Jeff Hayes