Is ‘Take something cum grano salis’ a popular phrase? Can I use it in casual conversation?

Cum grano salis is the Latin version of the phrase "take it with a grain of salt". This phrase means:

(With) a grain of salt, in modern English, is an idiom which means to view something with skepticism, or to not take it literally.

There is an interesting explanation to it, which says:

Since in Italy "to have salt in your pumpkin" (avere sale in zucca - pumpkin is a humorous way to say "head") means to have intelligence and reasoning capabilities, "grain of salt" often means "a little bit of intelligence". So, "cum grano salis", in its Latin form, it is often used when it is needed to show that intelligence and personal judgment are needed, as in "I drink wine cum grano salis since I must drive" (with care, moderately) or "please, repair this electric cable cum grano salis" (not scanting, thinking to the consequences or dangers of repairing your electric cable).

The phrase "take it with a grain of salt" is a rather common idiom in English. According to this Google NGram, the Latin form was originally more popular but around 1900 the trend switched to favor the English.

Nowadays, I had never heard the Latin phrase until you posted it. The English version seems to have mostly taken over. I think that using the Latin in conversation would lead people to believe that you were deliberately trying to seem learned, which might come off as disingenuous. The blogger you quoted uses the Latin phrase in a few different articles, but unless you're talking to professors or academics I wouldn't use it in conversation.

"Take it with a grain of salt" is definitely in common usage in normal speech, and everyday life, but if you were to say cum grano salis in a conversation, everyone's head is going to blow towards you, and everyone will be like, "What on earth is that new word?"

Basically, it's not in common usage at all, I wouldn't understand it if someone said it to me, I've never heard it being used. In writing, perhaps it would have been used, but definitely not in conversations and everyday life.

It's like saying mea culpa instead of "My fault", or "My bad.". People would understand "My fault", but not mea culpa. (Or, an alternative, saying "exempli gratia" instead of "for example" or "e.g.". The layman wouldn't understand you.)

Orwell's fifth rule of effective writing applies: "Never use a foreign phrase, scientific word or jargon word if you can think of an everyday English equivalent."

It's interesting that so many people think of Latin as a foreign language; they may be surprised how much of the English language is still Latin!

It is of course unkind to use language to make others aware of their own educational shortcomings. But when someone does, it reveals more about the lack of education of the person who uses the phrase, than those who don't understand it (they can always look it up somewhere). You may be aware that there are many similarities between modern English and modern Italian, I'd like to mention two now; Education and Educazione for example, and you would be forgiven for imagining these to have the same meaning. But you'd be wrong too. We know the English meaning, but the Italian means good manners.

An auntie of mine sometimes used a Latin phrase or two to add colour to a comment and I remember one in particular now. She would say "Idem con patate". it's a funny way of saying both things are the same.

Recently, we seem to be hearing more and more people calling for 'foreign words and phrases' such as Latin ones to be dropped from our language. I think this is a terrible mistake that will only add to a general ignorance of the language used people with a wider vocabulary. Every language has a history, and a future, those two are linked, not separate; you wouldn't cut off your left arm simply because you use your right one more, would you?