This idiom or expression has to do with:

Assuming that one will continue to have good luck.

You’re pushing your luck by fooling around with your boss’s wife. You’ve been lucky so far, but it is too risky.

Also:
Pressing one’s luck.

Also:
Pushing it.

Related warning:
Don’t push it!
Don’t press it!
Don’t press your luck!

Look, you’ve been lucky so far, but don’t push it!