The United States capital of Washington, D.C., stands for “District of Columbia,” but there is another jocular explanation—“Dot Com.”
     
“TEACHER ASKS JOHNNY IF HE KNOWS WHERE THE CAPITAL OF THE USA IS. JOHNNY SAYS YES IT IS WASHINGTON D.C. TEACHER SAYS FINE BUT DO YOU KNOW WHAT THE D. C. STANDS FOR. JOHNNY SAYS DOT COM” was posted on the newsgroup free.jokes on December 30, 1999. 
   
Other “D.C.” nicknames include “District of Confusion” (since at least 1943), “District of Corruption” (since 1951), “Washington Demands Cash” (since 1958), “District of Criminals” (since at least 1992) and “District of Cunts” (since 2009).
       
       
Wikipedia: Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington or D.C., is the capital of the United States of America. Founded after the American Revolution as the seat of government of the newly independent country, Washington was named after George Washington, first President of the United States and Founding Father.
     
Google Groups: free.jokes     
JOKE
stan patterson
Dec 30, 1999, 3:00:00 AM
TEACHER ASKS JOHNNY IF HE KNOWS WHERE THE CAPITAL OF THE USA IS. JOHNNY SAYS YES IT IS WASHINGTON D.C. TEACHER SAYS FINE BUT DO YOU KNOW WHAT THE D. C. STANDS FOR. JOHNNY SAYS DOT COM.
       
Google Grops: alt.callahans
Puns of the Weak 2-11-00
Stan Kegel
Feb 12, 2000, 3:00:00 AM
A teacher asked one of her pupils, “Can you name our nation’s capital?” The reply was, “Washington, DC” When asked what the “DC” stood for, the pupil added, “Dot com!” (Susie Mundy)
     
Google Groups: aus.jokes   
Washington DC
JokesnFun.com
Jul 25, 2001, 5:49:15 PM
(...)
A teacher asked one of her pupils,
“Can you name our nation’s capital?”
 
The reply was,
“Washington DC”
 
When asked what the “DC” stood for, the pupil added,
“Dot com!”
     
Google Groups: rec.humor
Washington Online
Mister - Funny - Bone - I n t e r n a t i o n a l
May 27, 2003, 6:07:26 PM
A teacher asked one of her pupils, “What’s
the nation’s capital?”
 
The reply was, “Washington DC”
 
On being asked what the “DC” stood for,
the pupil added, “Dot com!”
       
Twitter 
Steven Winterburn
@5tevenw
I asked my son ‘Whats the capital of USA?’ - ‘Washington DC.’ - ‘What does DC stand for?’ -  ‘Dot com!’
11:57 AM · Feb 16, 2010·Twitter Web Client
 
Twitter
Emeka Amakeze Fans
@TeamEmekaAmakez
Teacher - what is the capital of USA?
Student - Washington DC
Teacher - what does DC stand for?
Student - Dot com!!
6:37 PM · Aug 1, 2011·Twidroyd for Android
 
Twitter
Manikarnika Lagu
@CashBlueprint23
Teacher: What’s the nation’s capital?
Pupil: Washington DC
Teacher: What does DC stand for?
Pupil: Dot com!
7:52 AM · Aug 28, 2015·Twitter Web Client
 
Twitter
bara boy
@minkyol
what does dc in washington dc stand for???
dot com???? demo crat?? doja cat??
3:28 PM · Oct 5, 2020·Twitter for Android