מרצד

In modern Hebrew, people say [it is actually more literary than spoken] "אורות מרצדים" - flickering lights. Where does the word מרצד come from?

The pasuk says [Tehilim 68-16]: למה תרצדון הרים. 

How do mountains flicker???

Answer: They don't! 

It means either "shake" [as happens when there is a גילוי שכינה] or "lowered".

This is one of hundreds and hundreds of examples of modern words that mean something different today than they meant in Tanach and Chazal.