conundrum
Concise Oxford English Dictionary © 2008 Oxford University Press:
conundrum /kəˈnʌndrəm/
▶noun (pl. conundrums) a confusing and difficult problem or question.
■ a riddle.
– origin C16 (in the sense ‘a crank or pedant’): of unknown origin.
'conundrum' also found in these Oxford entries:

