Q From Ken Blose (Dennis Montgomery posed a related question): While serving as a tour guide at a museum, an English lady told me that, in the days of castles, the very finest shoes were made of dog skin. So if you were invited to the castle for a party or event, you would dress in your finest, and for shoes, you would put on the dog, meaning shoes made of dog skin!
A Inventive, these English. Firstly, to put on the dog (or to put on dog, in the form I learnt it) is first recorded only in 1871, in a book by L H Bagg called Four Years at Yale: “Dog, style, splurge. To put on dog, is to make a flashy display, to cut a swell”, and is certainly a US expression. So there’s really no chance at all of an English medieval origin. It has been suggested that it developed out of the rise in popularity of ladies’ lap dogs in the period after the American Civil War. Such animals were presumably pampered and beribboned, and this might have suggested that to put on the dog was to show off. This has the ring of a story made up after the event, but it’s the only explanation I’ve come across.