“FLOTUS” is an acronym for “First Lady Of The United States.” The term “FLOTUS” has been cited in print since at least 1983, when it referred to first lady Nancy Reagan..
   
Similar acronyms include “POTUS” (President Of The United States), “PEOTUS” (President-Elect Of The United States), “VPOTUS” (Vice President Of The United States), “SCOTUS” (Supreme Court Of The United States), “COTUS” (Constitution Of The United States), “COTUS” (Cat Of The United States), “DOTUS” (Dog Of The United States) and “TOTUS” (Teleprompter Of The United States).
     
 
20 September 1983, Washington (DC) Psot, pg. C1:
To their Secret Service shadows they may be “POTUS” and “FLOTUS,” but to each other out on their 688-acre California ranch he’s still her “Ronnie” and she’s still his “Nancy.”
 
9 April 1990, Electronic Buyers’ News, pg. 42:
The Secret Service has begun referring to President Bush as “Potus” and Mrs. Bush as “Flotus”—for President and First Lady of the United States.
     
27 December 1992, Atlanta (GA) Journal-Constitution, pg. C1:
He couldn’t make it and Flotus (the First Lady) filled in.
       
New York (NY) Times
ON LANGUAGE; It’s Him, All Right
By William Safire
Published: August 29, 1993
(...)
In the sentence “I am Flotus,” so useful in establishing the acronymic identity of the First Lady of the United States, I is the subject, am is the linking verb and Flotus is the predicate nominative.
 
26 September 1993,

(Baton Rouge, LA), “Walking the halls of power”:
“They call the President POTUS (President of the United States),” Scott said “The first lady is FLOTUS and the vice president is VPOTUS.”
 
New York (NY) Times
ON LANGUAGE; OFL Truth: At This Point in Timeline
By WILLIAM SAFIRE
Published: May 28, 1995
(...)
(Potus is the acronym for the initials of President of the United States; Flotus is for First Lady of the United States.)
 
Newsweek magazine
On The Road With Potus
Feb 25, 1996 7:00 PM EST
AS REPUBLICANS HURLED mud at each other in New Hampshire, Bill Clinton jetted around the country looking presidential. His advance team has special lingo:
 
POTUS: President of the United States. Also VPOTUS (Vice President), FLOTUS (First Lady).
   
New York (NY) Times
On Language; Potus And Flotus
By William Safire
Published: October 12, 1997
Controversy was bestirred by a memorandum dated Nov. 20, 1995, from high officials of the Democratic National Committee to Harold Ickes, then deputy chief of the White House staff, asking for ‘‘18-20 calls by POTUS’’ and ‘‘10 calls by VPOTUS.’‘
(...)
Flotus (pronounced FLOW-tus, to rhyme with Potus, and not FLOT-tus) is ‘‘First Lady of the United States,’’ an informal designation first applied to Mary Todd Lincoln that has become a quasiofficial title.
 
New York (NY) Times
Acronym
By BEN ZIMMER
Published: December 16, 2010
(...)
In English, the first known acronyms (as opposed to plain old initialisms) cropped up in the telegraphic code developed by Walter P. Phillips for the United Press Association in 1879. The code abbreviated “Supreme Court of the United States” as SCOTUS and “President of the…” as POT, giving way to POTUS by 1895. Those shorthand labels have lingered in journalistic and diplomatic circles—now joined by FLOTUS, which of course stands for “First Lady of the United States.”