CVSTOS RERVM PRVDENTIA

In The Decline of Classical Languages Bill Posner at Language log quotes Steve Berry's The Alexandria Link (pp. 418-419):

These words were chiseled into the granite below.

CVSTOS RERVM PRVDENTIA

"Prudence is the guardian of things," he said, translating, but his Greek was good enough to know that the first word could also be read as "wisdom". Either way, the message seemed clear.

Admittedly I don't know what CVSTOS RERVM PRVDENTIA means. I do get that it's Latin and not Greek, because I know rerum is the Latin thing. I have that much Latin, and also guess English prudence may genuflect toward prudentia in some way. Custos, though, at first glance I'm not sure what it leads to in meaningful English, or even what that sentence would make of itself if my first guess were substituted, leaving me with the phrase C[custom]R[thing]P[prudence] to puzzle over. "Custom thing prudence" is not immediately a sensible sentence in English, and whatever its meaning, CVSTOS RERVM PRVDENTIA was put there to make some sense to passersby by its makers, having taken the trouble to engage the chislers' craft to spell all the words right and line them up there just so on the rock.

In the other book I look to see if CVSTOS RERVM PRVDENTIA is a common Latin phrase, and find that custos is guardian or watchman, a word in Latin whose sound heads for custom in English, but is in fact the name for the other thing entirely. My surmise about custom was wrong, and I can discard all the bother about trying to make sense of what "Custom thing prudence" might entail in English.

But still, that leaves C[watchman]R[thing]P[prudence], and "Watchman thing prudence" doesn't offer up much of a sentence to my kind of English either.

When I first looked at CVSTOS RERVM PRVDENTIA I had zero confidence in my knowledge of the meaning of the word custos, and, say, 95% confidence in my knowledge of the meaning of the word rerum, and as for prudentia, well, I have to admit it's a coinflip between prudence and prudent, the noun of the quality or its readily applicable adjective for all I know, leaving me with at most middling confidence that my initial substitution was appropriate. I went with the noun prudence for no good reason. But if prudentia shades toward prudent then C[watchman]R[thing]P[prudent] says "Watchman thing prudent," and that, if spoken in the stiff-voiced phrasings of Frankenstein's monster, which for all I know was the Roman way as well, would be a readily understood if admittedly gothic way of warning the public to keep an eye out in English.

My confidence in my knowledge of the meaning of custos is way up, having found a quick fact about it, athough my confidence in my knowledge of the meaning of rerum is greater still, having here and there seen the confirmed thing in rerum this and rerum that many times over the decades. As for prudentia, I'll admit to the sort of equivocation in my confidence that only further study might resolve, not that I will.