A popular joke about legal definitions is:
 
Q: What’s the difference between “unlawful” and “illegal”?
A: “Unlawful” means “against the law.” “Illegal” (ill eagle) is a sick bird.
 
The joke has been cited in print since at least April 1957, when it was being told in Chicago. A New York City columnist noted in June 1957 that the joke was a “big gag at Lindy’s” restaurant. 
 
“Healthy eagles come from America. Ill eagles come from Mexico” is another “ill eagle” joke.
   
   
5 April 1957, Chicago Heights (IL) Star, “The Outlet,” pg. 4, col. 6:
Magistrate candidate Walter Baker in Park Forest insists he’s the only qualified attorney in the race and goes on to prove it by stumping opponents with, “What’s the difference between unlawful and illegal?”
 
Privately he advises his constituents that unlawful means against the law; illegal is a sick bird.
THE ADMIRAL
 
9 June 1957, Aberdeen (SD) American-News, “It Happened Last Night” by Earl Wilson, pg. 4, col. 5:
BIG GAG AT LINDY’S: “What’s the difference between ‘unlawful’ and ‘illegal’?”...“Well, ‘unlawful’ means contrary to law. ‘Illegal’ is a sick eagle.”
 
Old Fulton NY Post Cards
26 November 1957, Herald Statesman (Yonkers, NY), “Col. Joe Bush Says,” pg. 22, col. 6:
Courthouse rock ‘n’ roll humor: “What’s the difference between ‘unlawful’ and ‘illegal’? Answer: Ill Eagle is a sick bird”. (Paging Jack Benny—Call for Mister Benny.)
 
Google News Archive
8 May 1959, St. Petersburg (FL) Times, “Good Morning” by Chuck Albury, pg. 3-B, col. 1:
Clearwater Police Chief Willie Booth says the difference between “unlawful” and “illegal” is that something unlawful is “against the law” while the other (ill eagle) is “a sick bird.”
 
Google News Archive
20 August 1959, Kentucky New Era (Hopkinsville, KY), “Try and Stop Me” by Bennett Cerf, pg. 4, col. 3:
Mel Dodson, of Atlantic City, likes to ask the kids who wander into Convention Hall, “What’s the difference between ‘unlawful’ and ‘illegal’?”  The answer, of course, is that “unlawful” means “against the law” and “illegal” is a sick bird.
   
Google Books
May 1962, Boys’ Life, “Think and Grin,” pg. 82, col. 4:
Doctor: What is the difference between unlawful and illegal?
Lawyer: There isn’t any.
Doctor: Oh, yes there is. Unlawful is against the law, and illegal is a sick bird. — Mike Doran, Denver, Colo.
   
Google Books
Courtroom Kicktales
By John G. May, Jr.
Charlottesville, VA: Michie Co.
1964
Pg. 292:
Unlawful and Illegal
Jim Minor: “What is the difference between unlawful and illegal?”
John May: “Dog if I know.”
Jim: “Something unlawful is a prohibited act. Illegal is a sick bird!”
 
Google Books
Funny You Should Ask:
How to Make Up Jokes and Riddles with Wordplay
By Marvin Terban
New York, NY: Clarion Books
1992
Pg. 12:
Law professor: What’s the difference between unlawful and illegal?
Student: Unlawful is against the law. Illegal is a sick bird!
 
Google Books
Man Walks into a Bar:
Over 6,000 of the Most Hilarious Jokes, Funniest Insults and Gut-Busting One-Liners
By Stephen Arnott and Mike Haskins
Berkeley, CA: Ulysses Press
2007
Pg. 237:
What’s the difference between unlawful and illegal? Unlawful means against the law, illegal is a sick bird.
 
Twitter
I.T. Confessions
‏@ITconfessions
What’s the difference between unlawful and illegal? Unlawful is against the law, illegal is a sick bird.
12:26 PM - 24 Mar 2007
 
Google Books
Best Ever Classroom Jokes:
Because some of us never grow up
By Mike Haskins
New York, NY: Pavilion Books
2015
Pg. ?:
Define the difference between unlawful and illegal?
Unlawful means against the law and illegal is a sick bird.