I have no idea of the origins of this word, so my understanding of it may be wrong, but where I come from its a phrase that means you're so completely tired that you're over tired or totally beat. I can only hguess its a reference to the fact that a lot of dogs seem to sleep a lot and thus appear to always be tired and in need of sleep.
I pulled a double shift last night and when I got home the neighbours were having a rowdy party with loud music and I couldn't get any decent sleep. I am already dog tired and I have another double shift tomorrow.
by PlaceHolder August 11, 2004