When Slim tells Curley that he’s been asking him "too often" and that he’s "sick of it," what is he referring to? What is Curley asking him, and why does it make Slim mad?

Curley is asking Slim, if Slim and Curley’s wife have a thing for each other/have been getting together in private. This makes Slim mad, because he feels Curley is targeting him. Curley often asks Slim this, and Slim always gives him the same answer, which is no. Which makes Slim “sick of it”.