all wet - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Completely wrong, mistaken, as in If you think you can beat the system and win at roulette, you're all wet. The original allusion in this expression is unclear, that is, how moisture or dampness is related to wrongness. [Slang; first half of 1900s]