Entry in progress—B.P.
 
“If you’ve gained weight and nobody wants to mention it, you are the elephant in the room” is a joke on the saying.
 
     
Wikipedia: Elephant in the room
“Elephant in the room” is an English idiom for an obvious truth that is being ignored or goes unaddressed. The idiomatic expression also applies to an obvious problem or risk no one wants to discuss.
 
It is based on the idea that an elephant in a room would be impossible to overlook; thus, people in the room who pretend the elephant is not there have made a choice. They are choosing to concern themselves with tangential or small and irrelevant issues rather than deal with the looming big one.
 
Origins
The Oxford English Dictionary gives the first recorded use of the phrase as The New York Times on June 20, 1959: “Financing schools has become a problem about equal to having an elephant in the living room. It’s so big you just can’t ignore it.”
 
This idiomatic expression was in general use much earlier than 1959. For example, the phrase appears 44 years earlier in the pages of a British journal in 1915. The sentence was presented as a trivial illustration of a question British schoolboys would be able to answer, e.g., “Is there an elephant in the class-room?”
 
Usage
The term refers to a question, problem, solution, or controversial issue that is obvious, but which is ignored by a group of people, generally because it causes embarrassment or is taboo. The idiom can imply a value judgment that the issue ought to be discussed openly, or it can simply be an acknowledgment that the issue is there and not going to go away by itself.
 
The term is often used to describe an issue that involves a social taboo, such as race, religion, sexual orientation, or even suicide. This idiomatic phrase is applicable when a subject is emotionally charged; and the people who might have spoken up decide that it is probably best avoided.
 
The idiom is commonly used in addiction recovery terminology to describe the reluctance of friends and family of an addicted person to discuss the person’s problem, thus aiding the person’s denial. It is sometimes invoked as a “pink elephant”, in reference to alcohol abuse.
 
(Oxford English Dictionary)
elephant, n.
orig. U.S. the elephant in the room and variants: a big problem or controversial issue which is obviously present but ignored or avoided as a subject for discussion, usually because it is more comfortable to do so.
[1959 N.Y. Times 20 June 19 Financing schools has become a problem about equal to having an elephant in the living room. It’s so big you just can’t ignore it.]
1984 M. H. TYPPO & J. M. HASTINGS (title) An elephant in the living room: a leader’s guide for helping children of alcoholics.
1996 Sunday Times (Nexis) 22 Dec., All the comment had missed the elephant in the room—and thus, despite the SIB’s mild reform ideas, something like Sumitomo could happen again.
   
Old Fulton NY Post Cards
22 May 1952, Rogersville (TN) Review, “Around Town,” pg. 8, col. 3:
Match Furniture To Size Of Room For Best Effect
Elephants in the living room?
 
Oversized furnishings in undersized rooms can give that effect, say U-T home furnishings specialists.
(...)
A knowledge of proportion can help homemakers furnish their homes beautifully for no more than might be spent for the “elephant in the living room” effect, say the specialists.
         
Google Books
Our Changing Constitution
By Charles Leedham
New York, NY: Dood, Mead
1964  
Pg. 91:
Doing so was as difficult as ignoring an elephant in the living room, but both the pro- and anti-slavery forces in Congress managed it. They pretended that the problem did not exist, hoping that this “elephant” would quietly go away.
   
Google Books
Interpersonal development.
Publisher: Basel, Switzerland : S. Karger, 1970-
Edition/Format: Journal, magazine : Periodical : English
Pg. 82:
David Aspy (1969) has used the term ‘elephant experience’ to refer to ‘experiences which we are unprepared to assimilate’, which are ‘too much for us’ — as coming home to find an elephant in the living room. 
   
Google Books
Journeys out of the body
By Robert A Monroe
Garden City, NY: Anchor Press
1973, ©1971
Pg. 32:
Turn away and forget about it? In this case, two factors negated that possibility. One was nothing more than curiosity. The second: how can one forget or ignore an elephant in the living room?
 
Google Books
Elephants in the living room, bears in the canoe
By Earl Hammond, Liz Hammond and Elizabeth Levy
New York, NY: Delacorte Press
1977
Pg. 27:
‘What’s that?’ my father asked.
“That’s my elephant in the living room,“I replied.
 
Google Books
Family Therapy:
Full length case studies
By Peggy Papp; American Orthopsychiatric Association.
New York, NY: Gardner Press
1977
Pg. 169:
Although the other man died some years ago, to this day nobody has ever said anything about this affair, which went on for years; Fred said it was like an elephant in the living room with everyone pretending it was not there.
 
18 October 1981, Boston (MA) Globe , “A good, single man gets harder to find” by Judy Foreman, Living, pg. 1:
Dont look now, but there’s a large gray elephant in the room and nobody’s talking about it.
 
2 April 1985, Syracuse (NY) Herald-Journal, “Odds are most drink drivers are alcoholics: by Michelle Farrell, pg. B3, col. 3:
“Alcoholism is the white elephant in the living room that everyone walks around, but no one mentions….Proclaiming April as Alcoholics Awareness Month gives everyone a license to talk about it,” Kivari said.
     
OCLC WorldCat record
An elephant in the living room
Author: Jill M Hastings; Marion H Typpo
Publisher: Minneapolis : CompCare Publications, ©1984.
Edition/Format: Book : English
   
OCLC WorldCat record
There’s an elephant in the living room.
Author: Andrea Satin; Lee Engdahl; Clamshell Press.
Publisher: Santa Rosa, Calif. : Clamshell Press, ©1984.
Series: Broadside (Clamshell Press), no. 15. 
Edition/Format: Book : Poetry : English
 
OCLC WorldCat record
An elephant in the living room : a leader’s guide for helping children of alcoholics
Author: Marion H Typpo; Jill M Hastings
Publisher: Center City, Minn. : Hazelden, 1994.
Edition/Format: Book : English