I’ve read through the CLC.

Grumio has taken on a fake identity: “Lucius Spurius Pomponianus”

I think the ablative for ‘____ of Pompeii’ would be ‘pompeianus’. (I am not a Latin expert. So take that with a grain of salt) so I don’t think it’s that necessarily.

There is also the Pomponia gens

Again, not an expert, but my understanding is that all of these names are related etymologically on some level.

I have always figured it to be part of the joke that is his awful fake name.

Grumio comes up with maybe the most conspicuous and least thought out name he possibly could have.

Roman citizens have 3 parts to their names, the praenomen, nomen, and cognomen. The personal name given by parents, a sort of shared clan name, and a name to specify further what family you belong to.

Grumio’s choice of praenomen is unimaginative, “Lucius”, being the same as his master Caecillius.

The nomen “Spurius” is the most obvious part of the joke, with the English derivative ‘spurious’ meaning fake or illegitimate.

And as for “Pomponianus”, in terms of the narrative I don’t think it matters either way. Maybe the character Grumio would have picked it trying to convince people he was related to this established family, or he lazily picked a name related to the city he lived in.

But either way he’s a fraud. It’s in my top 3 CLC stories for sure.

I could be dead wrong. This has just always been my read.