In Louisiana, the preferred spelling is “phydeaux”. Among members of the United States Marine Corps (currently serving or not, there are no “ex-Marines” SIR) , the popularity of the name is obvious.
I once had a gecko named Fido, if that helps.
Oh, sure, that’s what they told poor old Honest Abe. “Yeah, that’s right, Abraham, we, uh, Fido is–uh–we sent him to live on a, a farm! A farm back home in Springfield! I mean, he wouldn’t have been happy–he wouldn’t be happy–living in Washington in that cramped old White House. So we sent him to live with, uh, the Joneses–you remember the Joneses, right?–on their farm, where he can run and play and chase things and stuff.”
My memory of school-boy latin is that the ending denotes the person.
Hence Fido means “I am faithful”
Fidus “you are faithful”
Etc.
Wasn’t there a dog collar labelled “Fido” found at Pompeii?
Or dog bowl, or sign, or something.
I recall seeing something in a National Geographic…
Your schoolboy Latin appears to have been mixed up with a little schoolboy Chinese. “Fido” may, indeed, be a verb, but the conjugation is:
*fido* I trust
*fidis* thou trustest
*fidit* he/she/it trusts
*fidimus* we trust
*fiditis* you trust
*fidunt* they trust
But the etymology of “Fido” is not directly Latin-to-English, and is not from the above verb. It is from fidus, the adjective:
Singular Plural
Masc. Fem. Neut. | Masc. Fem, Neut.
*fidus* *fida* *fid* | *fidi* *fidae* *fida* faithful
*fidi* *fidae* *fidi* | *fidorum* *fidarum* *fidorum* of faithful
*fido* *fidae* *fido* | *fidis* *fidis* *fidis* to faithful
*fidum* *fidam* *fidum* | *fidos* *fidas* *fida* faithful (direct object)
*fido* *fida* *fido* | *fidis* *fidis* *fidis* from faithful
For unknown reasons, the -o form of masculine (and neuter) became the normal masculine form in both Italian and Spanish when they evolved from Latin, and English “Fido” comes from one of them.
About six years ago, on a routine visit to the vet, my wife and I were in line behind a middle aged woman and her dog. While we waited to sign in, the woman was practicing obedience commands with the young dog. After each command/response, she squeaked a little ball and let the little guy give it a bite; followed by a rub and a ‘Good Boy’ or a ‘Good Boy Fido’. When we reached the desk, my wife drew my attention to the checkin sheet where the woman had written down her dogs name. It was written ‘Phydeaux’. Seriously funny at the time, and still a cute story.
Bill, Seminole FL
Bumped.
Proof that there’s a book about everything about Lincoln: http://www.olddurhamroad.com/product/6079/Book
Zombie thread but …
I was in the Phillips Collection in DC, sitting in front of Renoir’s “Luncheon of the Boating Party”. A guide leading a tour came in and I listened to her spiel. She said the woman was holding the dog because of the common name for a dog of Fido meaning faithful. The ironic part is that she said that the woman was Renoir’s mistress.
I can’t vouch for the accuracy of the story itself but that was the gist of the tour guide’s comment on the dog.
Jan Van Eyck’s famous painting Arnolfini Wedding Portrait (1434) is described as a pictorial wedding certificate, full of symbolic tokens. See, e.g., Jan Van Eyck and the Arnolfini Wedding Portrait, which shows the painting (scroll down a bit) followed by close-ups of a bunch of the symbols in detail, with discussion of each.
One of the items in the picture is a little doggy, which is described as a symbol of fidelity. Or maybe it was just a family pet that happened to be there.
My wife being blind from birth, a fairly large percentage of our circle of acquaintances are also blind (also, we live two blocks from the local campus of the Braille Institute). Anyway, as a consequence of this, we are acquainted with a high number of guide dog users. One of them has a dog named “Fido.”
She insists on pronouncing it “Fee-do.” Either because he constantly demands that she feed him, of because she identifies as an ancient Roman (no, I haven’t asked).
It isn’t. When signing clients up I joke that, because the Xeroxed screen print of a bogus account shows it as an answer for a security question, I assume her pet’s name is “Fido.” I actually get laughs from that, just like, based on her email address, I say, “And my psychic powers tell me your last name is ‘smith74?’” Women my age, especially from Down South, were trained to laugh at whatever stupid joke a man makes. And react positively. Cow-orkers from up here can’t pull it off, and wouldn’t think to try.
I’m murder on women fifty and up. 
The crux of the biscuit…is the apostrophe!