put on the dog. Get dressed up, to the nines, I would imagine.
The Morris Dictionary of Word and Phrase Origins says that this used to be a bit of a put-down, sort of like putting on airs. The Morrises go on to suggest that the dog may be one of those used as a fashion accessory, like a lap dog, perhaps. I suspect this is a variation of the phrase “all dogged up” cited in the Bloomsbury Dictionary of Contemporary Slang. There the suggestion is that it is “inspired by” “dolled up” or “decked out.”
Michael Quinion even gives us a first citation: “Dog, style, splurge. To put on dog, is to make a flashy display, to cut a swell,” from Four Years at Yale by L.H. Bagg. Quinion doesn't like the lap dog theory, but admits he can't offer anything better.
A variation of this might be seen in the use of the adjective “doggy” to describe one who is dashing or stylish, according to the OED. However, I doubt you will find this often in conversational English in the U.S.
1. Morris, William, and Mary Morris. 1988. Morris Dictionary of Word and Phrase Origins. 2nd ed. New York: Harper & Row.
2. Thorne, Tony. 1990. The Dictionary of Contemporary Slang. 1st American ed. New York: Pantheon Books.
3. Quinion, Michael. 1999. Put on (the) Dog. World Wide Words. Accessed Sep 20 2001 from
4. The Oxford English Dictionary Online. 2005. 3d ed. Accessed from
http:// www.quinion.com/ words /qa/ qa-put1.htm.