know somebody/something backwards
know something backwardsmainly BRITISH or
know something backwards and forwardsmainly AMERICAN
If you know a subject backwards, you know a lot about it. I thoroughly enjoy lecturing and know my subject backwards. He will be very much in demand. He knows the business backwards. They know company personnel policy backwards and forwards.
know something backwardsbe entirely familiar with something.
1991 William Trevor Reading Turgenev People who lived in the town knew it backwards.