An obvious problem, controversial issue, or uncomfortable truth that everyone is aware of but nobody wants to discuss or acknowledge.
This phrase gained popularity in the 1950s, though similar expressions existed earlier. It draws on the absurdity of ignoring something as large and obvious as an elephant in an indoor space, emphasizing how ridiculous it is to pretend not to notice significant issues.
The phrase brilliantly exploits our understanding that elephants are not only enormous but also highly intelligent and emotionally complex creatures - making their presence impossible to ignore on multiple levels. Ivan Krylov's 1814 fable about a man noticing small objects while missing an elephant helped establish this metaphorical framework.
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