“Due diligence” is a term that dates from at least 1598 in the Oxford English Dictionary, meaning “appropriate, sufficient, or proper care and attention.” “Do Your Own Due Diligence” (DYODD) means that one should never take someone’s advice or offer without properly investigating it first.
 
“Do your own due diligence” has been cited in print since at least 1988. “DYODD” has been cited in print since at least 2000, when it was cited in a penny stock bulletin board list of Internet and financial slang.
     
 
Wikipedia: Due diligence
“Due diligence” is a term used for a number of concepts, involving either an investigation of a business or person prior to signing a contract, or an act with a certain standard of care.
 
It can be a legal obligation, but the term will more commonly apply to voluntary investigations. A common example of due diligence in various industries is the process through which a potential acquirer evaluates a target company or its assets for an acquisition.
   
Etymology
The term “due diligence” first came into common use as a result of the United States’ Securities Act of 1933. This Act included a defense at Section 11, referred to as the “Due Diligence” defense, which could be used by broker-dealers when accused of inadequate disclosure to investors of material information with respect to the purchase of securities.
 
As long as broker-dealers exercised “due diligence” in their investigation into the company whose equity they were selling, and disclosed to the investor what they found, they would not be held liable for non-disclosure of information that was not discovered in the process of that investigation.
 
(Oxford English Dictionary)
due diligence n. chiefly Law appropriate, sufficient, or proper care and attention, esp. as exercised to avoid committing an offence; (Business, orig. U.S.) a comprehensive appraisal of a business undertaken by or on behalf of a prospective buyer, esp. in order to establish the exact scope of current assets and liabilities, and to evaluate future commercial potential.
1598   R. Carew Herrings Tayle sig. C2v,  That as the mindes like Monarchs, with due diligence Had their directions in euery part obeyd.
1666   M. Wigglesworth Day of Doom 89   Then will I draw all sorts of men to me, Oh, wait upon him with due diligence.
1789   Pennsylvania Gaz. (Electronic text) 2 Feb.,  If any Sheriff or Sheriffs, to whom any such writ or writs of capias shall be directed and delivered, shall refuse or neglect to use due diligence to execute the same, he and they shall severally forfeit and pay to the Mayor, Aldermen and citizens aforesaid, the sum of [etc.].
1820   Ohio Laws 18 xxiv. 164   [The endorsee] may..institute and maintain, an action on such bond, note or bill, for the recovery of the money due thereon..having first used due diligence to obtain the money of the drawer, maker or obligor.
 
4 October 1988, Lawrence (KS) Journal-World, “Seed capitalists give pointers at seminar” by Gwen Knudsen, pg. 6B, col. 5:
“You have to do your own due diligence on that venture capital firm,” Campbell said.
(Sam Campbell, president of Campbell-Becker, Inc.—ed.)
 
Google Books
Street Smart Networking:
A no-nonsense guide for creating the most richly rewarding lifestyle you can possibly imagine
By Robert Butwin
Charlottesville, VA: MLM Pub.
1994
Pg. 86:
Do your own due diligence and make sure you know exactly how the particular program will work for you.
 
Google Books
Financing Your Business
By Iris Lorenz-Fife
Englewood Cliffs, NJ: Prentice Hall
1997
Pg. 150:
DO YOUR OWN DUE DILIGENCE
 
Google Groups: alt.invest.penny-stocks
FOR NEWBIES: INTERNET AND FINANCIAL SLANG
Marc Lafargue  
1/28/00
(...)
Due Diligence = Research the fundamentals of a stock.
DYODD = Do your own due diligence.
 
Silver Values
Should You Buy Precious Metals Like Silver and Gold?
3 Apr, 2008
(...)
That is why you should enter a market like this with money you can afford to play with, and of course, DYODD (Do Your Own Due Diligence).
 
IKN
8/1/08
The value of good due diligence
DYODD*, dude!
(...)
*Do Your Own Due Diligence
     
Vancouver Venture
Tuesday, September 27, 2011
Cheap stock and doing your due diligence DYODD
Most people always get a hot stock tip from some trusted word of mouth source. Dollar $$ signs quickly appear in their eyes as they think they have the next $0.50 stock that is going to $3. Newsletter touts sell greed as slick promotional campaigns are put together and distributed en masse.
 
Google Books
The Smart Consumer’s Guide to Good Credit:
How to Earn Good Credit in a Bad Economy
By John Ulzheimer
New York, NY: Constable & Robinson
2012
Pg. ?:
DYODD [H11005] Do Your Own Due Diligence. I have a friend who sends me stock tips from time to time, and he always ends his emails with DYODD. What that means is: “Don’t complain if you buy a dog.” That same acronym applies to bank products.
 
Twitter
Alex Khalil‏
@AlexKhalil2  
DYODD = Do Your Own Due Diligence
6:04 PM - 21 Oct 12