I'm French study beginner, so I don't have so much knowledge about French but I'm trying to read a book, in which I found the sentence “c'est pas d'sa faute”.
What does “d'sa” mean in this sentence and why is it not “ce n'est pas d'sa faute”?
I'm assuming that sentence was part of a dialogue. The character is speaking in a familiar register of French, so the author is not writing in proper French to get that fact across.
In oral familiar French, it is common to drop the final e and kind of glue the next word to it. So d'sa is actually de sa elided together.
It is also very common to drop the double particle negative in oral French and omit ne altogether. You are correct, it should be Ce n'est pas, but with the ne omitted, it does become C'est pas.
There are two questions here.
So this is a common realisation of « ce n'est pas de sa faute » in oral French.
It's not his/her fault! But it's a very familiar expression, the correct one would be ”Ce n'est pas de sa faute”.