Just like the English language, French has many idioms and expressions involving animals. Some of these have relatively similar English equivalents, whereas others have seemingly nonsensical direct translations! Either way, learning these idiomatic expressions is a great way to understand more of the French that you read and hear, and speak more like a native.
rire comme une baleine

Meaning: to laugh one’s head off, to roar with laughter
Lit.: to laugh like a whale
Example: Quand elle m’a raconté l’histoire, j’ai ri comme une baleine. When she told me the story, I laughed my head off.
Note: you might also see se marrer comme une baleine as a variation of this idiom. (Se marrer is just an informal word for rire.)
être fait comme un rat

Meaning: to be cornered, to have no escape, to be trapped
Lit.: to be trapped like a rat
Example: Dès que la police a encerclé la maison, le suspect était fait comme un rat. As soon as the police surrounded the house, the suspect was trapped.
Note: fait can be used as an informal word for “trapped”.
être serrés comme des sardines

Meaning: to be packed like sardines, to be crowded very close together
Lit.: to be packed like sardines
Example: Les voyageurs étaient serrés comme des sardines. The passengers were packed like sardines.
sauter du coq à l’âne

Meaning: to jump from one subject to another, to suddenly change the subject
Lit.: to jump from the rooster to the donkey

Example: Il saute du coq à l’âne sans arrêt. He constantly jumps from one subject to another.
Note: you may see this idiom used with passer instead of sauter. Both variations have the same meaning.
avoir d’autres chats à fouetter

Meaning: to have other fish to fry
Lit.: to have other cats to whip
Example: Je ne peux pas sortir ce soir, j’ai d’autres chats à fouetter. I can’t go out tonight, I’ve got other fish to fry.
être rusé comme un renard

Meaning: to be as cunning as a fox, to be sly
Lit: to be cunning like a fox
Example: Je ne ferais pas confiance à Bruno, il est rusé comme un renard. I wouldn’t trust Bruno, he’s as sly as a fox.
quand les poules auront des dents

Meaning: pigs might fly, when pigs fly
Lit.: when hens have teeth
Example:
<< Un jour, je serai un millionnaire. >>
<< Oui, quand les poules auront des dents ! >>
“One day, I’ll be a millionaire.”
“Yes, and pigs might fly!”
avoir un chat dans la gorge

Meaning: to have a frog in one’s throat, to be hoarse
Lit.: to have a cat in one’s throat
Example: Il a eu un chat dans la gorge pendant son discours. He had a frog in his throat during his speech.
temps de chien

Meaning: dreadful weather
Lit.: dog’s weather
Example: Il fait un temps de chien ! It’s dreadful weather!
revenons à nos moutons !

Meaning: let’s get back to the topic at hand, let’s get back on track (after a digression)
Lit.: let’s come back to our sheep!
Example: << Nous avons encore changé de sujet ! Alors, revenons à nos moutons ! >> “We’ve changed the subject again! Right, let’s get back on track!”
parler le français comme une vache espagnole

Meaning: to speak poor French, to speak French really badly
Lit.: to speak French like a Spanish cow
Example: Je ne peux pas du tout le comprendre, il parle le français comme une vache espagnole. I can’t understand him at all, his French is awful.
en parlant du loup

Meaning: speaking of the devil
Lit.: speaking of the wolf
Example: << Tiens, en parlant du loup, voilà Paul qu’arrive. >> “Hey, speaking of the devil, here comes Paul now.”
Note: in the same way that we say “speak of the devil” in English, you can also say quand on parle du loup in French.
The “full” expression is quand on parle du loup, on en voit la queue (Lit.: when we speak of the wolf, we see its tail.), or en parlant du loup, on en voit la queue. However, people don’t tend to say the second clause. Again, this is similar to English. The “full” expression is “speak of the devil, and it shall appear”, but we just tend to say “speak(ing) of the devil.”
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