off-the-wall

1. Very unconventional or unusual: manic, off-the-wall creativity; off-the-wall humor.
2. Exhibiting bizarre behavior; crazy: their off-the-wall friends.

Eccentric, unconventional, as in That idea of opening a 100-seat theater is off the wall. This expression probably originated in baseball or some other sport in which the ball can bounce off a wall in an erratic way. [Colloquial; 1960s]