1In plural The hottest part of the summer, associated in ancient times with the heliacal rising of the Dog Star in the Mediterranean area, and formerly considered to be the most unhealthy period of the year and a time of ill omen.
2A day in the dog days.
3figurative In plural In early use: an evil time; a period in which malignant influences prevail. Now: a period of inactivity or decline.
Mid 16th century; earliest use found in Thomas Elyot (c1490–1546), humanist and diplomat. From dog + day, after post-classical Latin caniculares dies dog days, itself after Hellenistic Greek κυνάδες ἡμέραι.