spitboiling

Pursuing a fruitless and pointless endeavor.

Spitting in a pot to boil it would be a waste of time and energy and would likely never succeed, as well as being a frivolous and senseless activity which achieves no purpose in the first place, hence the term spitboiling.

"My mind is made up; you're just spitboiling trying to convince me."
"We haven't made any progress. We need to stop spitboiling and either change our approach or give up."
"Is there a reason you're doing that or are you spitboiling?"