nail Jell-O to the wall
nail Jell-O to the walland nail Jell-O to a cross and nail Jell-O to a tree
phr. to do something that is totally futile. (Jell-O is a protected trade name.) You’re wasting your time. Trying to get him to do that is like nailing Jell-O to the wall.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.