Learner's definition of DEUCE
1 [count], chiefly US, informal : a playing card that has the number two on it or two symbols on it 2 tennis : a situation in which each side has a score of 40
3 [singular], old-fashioned + informal — used to make a question or statement more forceful
Who the deuce [=devil, hell] was that?
We've had a deuce of a time finding a good doctor. [=it has been very difficult to find a good doctor]