？
Can I use your bathroom?
Literal
Toilet [obj-を] honorific-borrow-[can-していい]?
Japanese euphemistically frames 'using the bathroom' as 'borrowing the toilet,' even in one's own home or at a restaurant — the metaphor softens the physical request. お借りする is the humble (kenjougo) form of 借りる 'to borrow,' formed with the お+stem+する humbling pattern. ~していい? is the casual 'is it okay if I do X?' permission-seeking question.