Dog-tired

Means utterly exhausted, resembling the way a dog flops down when it has over-exerted itself. The expression dates from the late 18th/early 19th century. An earlier form of the expression is 'dog-weary' used by Shakespeare in The Taming of the Shrew, Act IV, Scene II, when Blondello says, "I have watched so long that I am dog-weary" which means that this version probavly dates from at least the mid-1500s.