(source: Iceberg Slim- Pimp, the story of mylife)

The section that contains the sentence:

”Preston, I don’t hip to the murder game. I don’t want to get hip to it. I don’t wnat to blow his brains out on that sidewalk and waste them. I want his brains to work inside my skull. You getting old, Preston. You can’t even dent the mustard. He screwed you around a thousand times worse than me. ”

Background:

An old friend called Preston gives to Slim a gun and tries to persuade him into killing Sweet Jones (the pimp King of the city) Slim refuses him and thinks that Preston wants to make him do the dirty work instead of him because of an old conflict between Sweet and Preston. Above are the words of Slim refusing Preston.

So what does ’You can’t even dent the mustard’ mean here?

Is it a common saying in English?