Dog Days

Everyone knows that the “dog days of summer” occur during the hottest and muggiest part of the season. Webster defines “dog days” as...

1 : the period between early July and early September when the hot sultry weather of summer usually occurs in the northern hemisphere
2 : a period of stagnation or inactivity

But where does the term come from? Why do we call the hot, sultry days of summer “dog days?”