To party with no inhibitions; i.e. unleash the beast. The phrase was originally made popular by the worst song of all time, "who let the dogs out?", but now is used mockingly.
Since the world is supposedly going to endtomorrow, I am going to let the dogs out tonight.
Used in politecircles when you are out on the lash need to go home for a poo. Best used with embellishment.
Brad: Sorry lads but I have got to go home and let the dogs out.
Angelina: I thought so, they have been barking all afternoon.
Brad: Yeah, if I don't go now there will be horrible mess.
While our lifespan is the number of years we will live, our healthspan is the number of healthy years we will have and our sexspan is is the number of years we will continue to be healthy enough to continue having an active sex life.