“Buck up, Bree,” said Cor. “It’s far worse for me than for you. You aren’t going to be educated. I shall be learning reading and writing and heraldry and dancing and history and music while you’ll be galloping and rolling on the hills of Narnia to your heart’s content.” “But that’s just the point,” groaned Bree. “Do Talking Horses roll? Supposing they don’t? I can’t bear to give it up. What do you think, Hwin?” “I’m going to roll anyway,” said Hwin. “I don’t suppose any of them will care two lumps of sugar whether you roll or not.”
It is not as common an idiom, at least among non-native speakers, as ‘care two hoots’ usually used in a negative sense. I think @Torsten will concur with me.
I did a quick Google check and did not find a different instance of this saying. Based on this and some of the other questions here, I think English Professor Lewis took more common phrases such as “I don’t give two hoots”, and changed a few words to adapt them for the land of Narnia.
I have never heard it before either but the meaning is clear and quite colourful. I particularly like Lawrence’s ‘two hoots’, which is more my cup of tea. A more vulgar expression to be avoided in ‘polite society’ is ‘I don’t give a monkey’s.’