It is often quite difficult to know where a language idiom is coming from (it is less etymology than social history). For English, I foudn the ideal web site: phrases.org.uk.
In their own words:
The meanings and origins of over 1,200 English sayings, phrases and idioms.
Whether you want to resolve a friendly argument over how a saying or phrase originated or whether you just enjoy words, you’ll probably find something here to interest you.
Now, if you are more interested in the French language, you can get the corresponding information from the reference web site: expressio.fr. There, the phrase of today is: