What a lawyer calls a “brief” if often very long. Jokes about this date to at least 1884.  “A lawyer calls it a brief and then makes it 96 pages in length” was printed in the Gold Leaf (Henderson, NC) on September 29, 1887.
 
“Willie, why does a lawyer write 160 pages of a legal document and then call it a ‘brief?’” was printed in the Brooklyn (NY) Citizen on August 18, 1903. “A lawyer is someone who can write a 10,000-word document, and call it a ‘brief’” was printed in the Burlington (VT) Free Press on June 7, 1963.
 
   
Newspapers.com
22 November 1884, Buffalo (NY) Commercial Advertiser, “All Sorts,” pg. 4, col. 7:
“What is that, lovey-dovey?” asked newly-married Mrs. Legalcap Demurrer of her husband, as he laid a heavy volume on the mantel.
“That is my brief in the Jones case, sweetie,” he responded, with a kiss.
“Why does lovey call it a brief?”
“Because it contains 386 pages.”—Washington Hatchet.
     
Newspapers.com
29 September 1887, Gold Leaf (Henderson, NC), pg. 1, col. 5:
A lawyer calls it a brief and then makes it 96 pages in length.
       
Newspapers.com
8 November 1887, Washington (NC) Weekly Progress, pg. 5, col. 2:
A lawyer calls it a brief and then makes it 66 pages in length.
 
Newspapers.com
18 August 1903, Brooklyn (NY) Citizen, “Current Humor,” pg. 4, col. 5:
“Willie, why does a lawyer write 160 pages of a legal document and then call it a ‘brief?’”
“That’s one on me, Sallie. But, then, there are a good many tins about the lawyer business I don’t understand.”—Roller Monthly.
 
Newspapers.com
1 October 1903, West Schuylkill Herald (Tower City, PA), “The Funny Side of Life,” pg. 2, col. 5:
WILLIE AND SALLIE.
“Willie, why does a lawyer write 160 pages of a legal document and then call it a ‘brief?’”
“That’s one on me, Sallie. But, then, there are a good many tins about the lawyer business I don’t understand.”—Roller Monthly.
 
Newspapers.com
14 October 1920, The Roanoke News (Weldon, NC), “Local Intelligence,” p. 3, col. 1:
A lawyer calls it a brief and then makes it 99 pages in length.
   
Newspapers.com
24 October 1941, Tampa (FL) Morning Tribune, pg. 10, col. 6:
Bob Burns says a lawyer is one who writes a thousand words, then calls it a brief.
BLANCHE HODGES
 
15 October 1957, Stamford (CT) Advocate, “On the Square” with Len Massell, pg. 14, col. 1:
Thought: A lawyer is a guy who’ll read a 10,000-word document and call it a “brief.”
 
Newspapers.com
7 June 1963, Burlington (VT) Free Press, “The Old-Timer Says,” pg. 16, col. 3:
A lawyer is someone who can write a 10,000-word document, and call it a “brief.”
   
Google Books
July 1980, Boys’ Life, “Think & Grin,” pg. 82, col. 3:
A lawyer is the only person who can write a 10,000-word document and call it a “brief.” — Doug Cooper, Zionsville, Ind.
 
Google Groups: alt.fan.mark-brian
Another Mark&Brian racist remark
Dave Marron
8/8/00
(...)
“A lawyer is someone who writes an 80-page document, and calls it a ‘brief’.”
- Alfred E. Neuman
     
Google Books
The Mammoth Book of One-Liners
Edited by Geoff Tibballs
London, UK: Constable & Robinson Ltd
2012
Pg. ?:
A lawyer is someone who writes a 90-page document and calls it a brief.
 
Twitter
Anna
@PompousGenuis12
A lawyer is someone who writes a 10,000 word document called a brief..  not so brief to me
12:57 AM · Apr 12, 2014·Twitter for iPhone
 
Twitter
Rick Brutti
@Rbrutti
A lawyer is someone who writes a 40-page document and calls it a brief!
Alfred E. Neuman
4:02 PM · Sep 14, 2016·Twitter Web Client
 
Twitter
Living Sarcasm
@SarcasmLiving
A lawyer is someone who writes an eighty-page document and calls it a brief!
1:02 AM · Jan 22, 2020·Twitter for Android