1538, from Latin dies caniculares, translated from Ancient Greek; originally from the hot summer days (in the Northern Hemisphere) when Sirius (the Dog Star), in Canis Major, rose and set with the Sun (heliacal rising). The Greeks also made reference to these "dog days", and for the ancient Egyptians, c.3000 B.C.E., the rising of this star coincided with the summer solstice and the start of Nile flooding. The "dog" association apparently began here, as the star's hieroglyph was a dog, a watchdog for the flooding of the Nile.
- The days between early July and early September when Sirius (the Dog Star) rises and sets with the Sun.
- hot, lazy days
- A period of inactivity, laziness, or stagnation.
Usage notes 
- "Dog days" have long carried an association as the most hot, stagnant, and unwholesome time of the year, usually July 3 to August 11, but variously calculated, depending on factors such as latitude, historical period, or whether the lesser star Procyon is also reckoned. Specifically, the heliacal rising of Sirius has shifted down the calendar with the precession of the equinoxes, making the exact dates of the "dog days" significantly different now from those in former times.