Definition from Wiktionary, the free dictionary
Alternative forms 
Perhaps from baseball or other games played with a ball, alluding to unpredictable bounces.
- (idiomatic) Wildly unconventional; bizarre; absurd.
- What kind of a harebrained, off-the-wall idea is that?
- She is a very off-the-wall character.
- (idiomatic) Greatly inappropriate.
- His behaviour today is off-the-wall.
Wildly unconventional; bizarre; absurd