Two cows walk into a milk bar and order watercress sandwiches. Alex DeLarge winks at his droogs and then walks over to the pair of heifers. "Excuse me," he begins:

And what do you think those two lovely cows said, me brothers?
"What's the shortest
English-language word
that contains four U's?"

What answer did the cows give him? 

Here is the SOLUTION. 

SOLUTION:

MUUMUU.