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.