from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Informal Very unconventional or unusual: manic, off-the-wall creativity; off-the-wall humor.
- adj. Informal Exhibiting bizarre behavior; crazy: their off-the-wall friends.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Wildly unconventional; bizarre; absurd.
- adj. Greatly inappropriate.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adj. conspicuously or grossly unconventional or unusual
Perhaps from baseball or other games played with a ball, alluding to unpredictable bounces. (Wiktionary)