Learn English – the origin of ‘cash’

Paul Beale, ed., Partridge's Concise Dictionary of Slang and Unconventional English (1989) offers this entry for the verb snog:

snog, v. 'To make love with repletion of kissing and cuddling; hence, snogging session, making love' (L.A. 1977); very common throughout WW2 [and still, 1983, not ob.: P.B.] I surmise a dial. alteration of snug, cosy, notably as in that snuggling-up which so often preludes a warmer conjunction. —2. Hence, to flirt, or to court, esp in be or come or go snogging: beatniks', adopted, ca. 1959 , ex gen s. (Anderson) The term, esp as be or go snogging, seems to have orig in the RAF, late 1930s (Partridge 1945).

However, Eric Partridge, A Dictionary of Slang and Unconventional English, Fifth Edition (1961) suggests a slightly different etymology:

snogging, be or go. To be or go courting a girl; to be or go love-making: RAF: since ca. 1937. Partridge 1945, 'Snog is perhaps a blend of snug and cod (to flatter or kid a person).'

That same source notes that "snogged up"—supposedly an RAF term dating to circa 1939—means "Smartened up, 'all dressed up.'"

The answer to your question is "Gare à l'eau!" or less commonly nowadays "Gare l'eau!" [=look out for the water!].

Actually I have never heard anyone say "gare quelque chose." The typical phrasing is "gare à quelque chose" [look out/watch out for something] in modern day French.

E.g.

"Gare à la voiture!" or even more commonly "Attention à la voiture!" [Watch out for the car!]"

That said, "Gare la voiture!" is grammatically correct and standard French, but is more likely to be found in literary works.

Besides, interjective "gare à" when addressed to someone is an invective exclamation. E.g. Gare a toi! [Watch out!]; Gare à ce que tu dis! [Mind/watch what you're saying!].

In conclusion, Farlex Trivia Dictionary knows best.

• Wiktionary "Garde à l'eau (or more commonly dans l'eau) lit. "guard in the water," the typical French expression being "homme à la mer," lit. "man in the sea" [man overboard]) = Guard overboard, but "Garde! A l'eau!" = Guard! (Get yourself) into the water! Plus, "Garde! A l'eau!" can also be understood as "Guard! Come get your water/Water's served," the typical French expression being "A la soupe!" [Soup's ready/served!].

• The Free Dictionary "Gare de l'eau" = The Water Station, but "Gare! De l'eau!" = Watch out! Water's coming!

• oxforddictionaries "Regarde l'eau" = Look at the water!

• unusuedwords.com "Garde de l'eau" = Spare some water (=don't use it all), or "Garde l'eau" = (You can) keep the water.

NB: In everyday speech, if you say to a French person "Gare la voiture" in a monotonous tone, chances are it'll be understood as "park the car," as long as there's also a verb, "garer" [to park], which is declined into "gare" in the present simple, the present subjunctive, and the imperative.

Edit: What actually is quoted in the Free Dictionary is not "gare à l'eau" [look out for the water], but "gare de l'eau," which still makes lots of sense in French provided the appropriate punctuation is added: "Gare de l'eau" [=The Water Station], but "Gare! De l'eau!" [Watch out/look out! Water's coming!]. Hence the most logic English transcription "Gardyloo!".