À l’eau, c’est l’heure!
In Cassell’s Dictionary of Slang by Jonathon Green (2nd edition, 2006), we find this set of related entries for cod:
cod, n.² 1 [mid–late 17C] a friend; thus honest cod, a good friend. 2 [late 17C–18C] a fellow. 3 [late 17C+] a fool. [(2) in this context cod has been linked to SE codger, and it is found as an abbr., but cod is a much earlier word; (3) ? ᴄᴏᴅ’ꜱ ʜᴇᴀᴅ n.]
cod, n.⁶ [20C+] (orig. Irish) 1. a joke, a hoax, a leg-pull, a parody. 2. deception, deceipt, a lie; thus, cod-acting foolish behaviour. [ᴄᴏᴅ v. (2)]
cod adj.¹ [1950s+] fake, parodic; usu. in combs., e.g. cod-Russian, code-typewriter etc. [ᴄᴏᴅ v. (2) + ? play on ꜰɪꜱʜʏ adj.³ (1); note theatrical jargon cod version, a burlesque of a well-known play]
cod v. 1 [18C] to cheat, to defraud. 2 [mid-19C+] (also codd) to tease, to hoax. [ᴄᴏᴅ n.² (3)]
The OED doesn’t seem to think it can come from codger, and neither does Green here.
I’m more familiar with the use of cod in this way for Latin than for French. If you search for cod Latin on Wikipedia, it brings up this page:
Dog Latin, also known as Cod Latin, macaronic Latin, mock Latin, or Canis Latinicus, refers to the creation of a phrase or jargon in imitation of Latin, often by "translating" English words (or those of other languages) into Latin by conjugating or declining them as if they were Latin words. Unlike the similarly named language game Pig Latin (a form of playful spoken code), Dog Latin is more of a humorous device for invoking scholarly seriousness.
That article also references the comic work Mots d’Heures: Gousses, Rames (viz. “Mother Goose Rhymes”) as being one deliberately written in “cod-French”. These are related to the phenomenon that has of late come to be called a mondegreen.
These mock versions of other languages have a long history. One fairly recent one, deriving from the early days of the computer age, is in cod-German and has come to be known as the blinkenlights meme; from Wikipedia:
ALLES TURISTEN UND NONTEKNISCHEN LOOKENPEEPERS!
DAS KOMPUTERMASCHINE IST NICHT FÜR DER GEFINGERPOKEN UND MITTENGRABEN! ODERWISE IST EASY TO SCHNAPPEN DER SPRINGENWERK, BLOWENFUSEN UND POPPENCORKEN MIT SPITZENSPARKEN.
IST NICHT FÜR GEWERKEN BEI DUMMKOPFEN. DER RUBBERNECKEN SIGHTSEEREN KEEPEN DAS COTTONPICKEN HÄNDER IN DAS POCKETS MUSS.
ZO RELAXEN UND WATSCHEN DER BLINKENLICHTEN.
This sort of macaronic language is used by Umberto Eco to good effect in various novels of his, famously including in Il Nome della Rosa (English: The Name of the Rose) from the mouth of his character Salvatore:
“Penitenziagite! Vide quando draco venturus est a rodegarla l’anima tua! La mortz est super nos! Prega che vene lo papa santo a liberar nos a malo de todas le peccata! Ah ah, ve piase ista negromanzia de Domini Nostri Iesu Christi! Et anco jois m’es dols e plazer m’es dolors... Cave el diabolo! Semper m’aguaita in qualche canto per adentarme le carcagna. Ma Salvatore non est insipiens! Bonum monasterium, et aqui se magna et se priega dominum nostrum. Et el resto valet un figo seco. Et amen. No?"
Since that’s macaronic Italian not macaronic English, it may be harder to see for those without a fair knowledge of Latin and its many daughter tongues that Eco is here weaving together. So here’s the would-be English translation from the English version of Eco’s novel:
“Penitenziagite! Watch out for the draco who cometh in futurum to gnaw your anima! Death is super nos! Pray the Santo Pater come to liberar nos a malo and all our sin! Ha ha, you like this negromanzia de Domini Nostri Jesu Christi! Et anco jois m’es dols e plazer m’es dolors.... Cave el diabolo! Semper lying in wait for me in some angulum to snap at my heels. But Salvatore is not stupidus! Bonum monasterium, and aquí refectorium and pray to dominum nostrum. And the resto is not worth merda. Amen. No?”