My boyfriend keeps saying 'I can't be arsed (with this)!' Is this proper English? Can I say this to his mum?
Thanks for your question.
'Can't be arsed' and 'can't be bothered' are used in the same way. 'Can't be arsed' is very common, but a bit stronger than 'bothered'. (Even stronger would be 'can't be f**ked').
So this is spoken English, although the Queen probably doesn't say it often.
Using it with his parents depends on what his parents are like, and how well you know them. However, in my opinion, 'arsed' is not too rude and I have definitely said it to my parents many times. In general I think I would use 'bothered', if I were you.
Hope this helps