1 Charon: This is where I leave you. One word of advice. Cave canem.
2 {Charon leaves in his boat}
3 Marcus: That was two words.
3 Julius: Cave canem? "Beware of the dog"?
4 Julius: What do you suppose he meant by that? {Cerberus, the giant three-headed guardian dog of Hades, looms behind Julius}

The Latin term cave canem is immortalised in many surviving Roman mosaics, such as this one in Pompeii.

The canis in question is of course Cerberus, the three-headed dog who guards the Gates of Hades.

And so begins the inevitable descent of Irregular Webcomic! into Cerberus Syndrome (scroll down a bit).

(And yes, I know, you don't need to e-mail me.)

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2015-08-21 Rerun commentary: The story I always heard of the original meaning of cave canem was that the dogs the ancient Romans had as pets were all very small breeds. The sign was intended to warn visitors that there was a dog on the premises so they could be careful not to step on it.

The most famous example of "cave canem" in surviving Roman artefacts is a mosaic in the House of the Tragic Poet in Pompeii. Preserved by the ash of the 72 AD Vesuvius eruption, the mosaic indeed shows what looks like an annoying little yappy dog.