1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): off-the-wall
Fig. odd; silly; unusual. • Why are you so off-the-wall today? • This book is strange. It’s really off-thewall.
McGrawhill's American Idioms And Phrasal Verbs