‘Out-With?’ asked Bruno. ‘What’s an Out-With?’ ‘It’s not an Out-With, Bruno,’ said Gretel with a sigh. ‘It’s just Out-With.’ ‘Well, what’s Out-With then?’ he repeated. ‘Out with what?’ ‘That’s the name of the house,’ explained Gretel. ‘Out-With.’
This is Bruno's mispronunciation of "Auschwitz", the major Nazi concentration camp in Poland