Ian B

I've known the term for over 40 years from my time in the "Andrew" where it meant being well dressed to go out for the evening. The standard for men was immaculate grey flannels, white shirt, tie, Dark Blue blazer and black shoes.
It may come from DB meaning Dress Blues which was sometimes used as an acronym on party invites.
Another contraction used was to describe someone, usually female, as "Just the Dogs", being considered a great compliment.
Of course "a dog" meant just that!
Hope this helps but who really knows where slang terms originate?