The Oxford English Dictionary gives the first recorded use of the phrase, ‘Elephant in the room’, as a simile, 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."
According to Wikipedia, "Elephant in the room" is an English metaphorical 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 chosen to concern themselves with tangential or small and irrelevant issues rather than deal with the looming big one
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, 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.