Mum

In chapter 4 of Great Expectations, at the time of Christmas festivities, at the house of Mrs Joe, she says to Pumblechook, "Mum, a bottle of sherry," and "Mum a bottle of port wine." 

Here the word Mum is a command to the children assembled in the house to keep quiet.  This is a nice piece of Dickensian characterization.  Mrs Joe controls the children including Pip in the house, and at the same time furnishes an information to Mr Pumblechook. 

Pip is explaining it retrospectively, and we see the entire scene through the eyes of frightened Pip.