A grim, watchful keeper, house-porter, guardian, etc. Cerberus, according to Roman mythology, is the three-headed dog that keeps the entrance of the infernal regions. Herculës dragged the monster to earth, and then let him go again. (See Sop.)
Orpheus (2 syl.) lulled Cerberus to sleep with his lyre; and the Sibyl who conducted Æneas through the Inferno, also threw the dog into a profound sleep with a cake seasoned with poppies and honey.
The origin of the fable of Cerberus is from the custom of the ancient Egyptians of guarding graves with dogs.
The exquisite cameo by Dioscoridês, in the possession of the King of Prussia, and the painting of Hercules and Cerberus, in the Farnésé Gallery of Rome, are of world-wide renown.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894