To pursue or continue with some futile task or activity, wasting one's time as a result. Usually used in the continuous tense. I've spent my life campaigning for environmental protection, but with the way the world's governments continue to favor corporations over everything else, I feel as though I've been plowing water this whole time. You're plowing water if you think you're going to change Henry's mind.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
n. wasting time doing something futile. You’re wasting your time. You’re plowing water.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.