polish a turd

polish a turd (third-person singular simple present polishes a turd, present participle polishing a turd, simple past and past participle polished a turd)

1. (idiomatic, vulgar) To work on a time-consuming and ultimately pointless or impossible task.
   • 2006, Michael Fak, Portions of a Life: Recollections and Reflections, p. 197:

     Demaso, trying hard to scrub a wall behind one of the toilets, started complaining how dirty the walls in the entire bathroom were. Tripp also chimed in that they were polishing a turd and that what the bathroom needed was a fresh coat of paint.

• you can't polish a turd