To form the negative in French, we usually use two little words, ne and pas. We place ne in front of the verb and pas after the verb. English mostly gets by with one word, not. For example:
French – je vais – je ne vais pas
English – I am going – I am not going
The word “pas” in French also has another meaning. It is a noun that means “step,” as in to take a step forward. If you are a ballet fan, you have heard of the expression “pas de deux” used to describe a dance for two people.
Linguists tell us that the use of “pas” as a negative came from people using pas in its original meaning of a step.
Just as in English we might say “I am not walking one step,” in French the expression at one time was “je ne march pas,” with the meaning of “I am not walking a step.”
This use of “pas” became extended to apply in other cases also, and so evolved into the present day use of “pas” in most negative sentences.
This process of turning a noun into a grammatical word is called “grammaticalization” by those who study the history and evolution of languages. The French use of “pas” is just one example of this.