I apologize for "vade mecum." In this case I can identify the precise source of the problem.
The furniture of our minds, including but not limited to our vocabularies, is strongly influenced by our most effortful spells of intellectual activity.
For me, that was the time thirty-odd years ago when I was trying, with only very limited success, to learn Chinese. Construing some text or other I came across the word ??, which I did not know. Consulting a Chinese-English dictionary I got "vade mecum." These things just stick in your mind.
Dictionaries compiled in foreign countries are often way behind the times usage-wise. One Chinese-English dictionary still used in mainland China translates ??? as "n****r brown." This got a Canadian furniture importer in deep trouble a few years ago.
While I'm on the subject, here is my favorite page from the 1979 ????, a popular Chinese-Chinese dictionary. Look at the extreme top right, next to the page number.