Literally: strange – thought – heavens – outside
Alternately: A truly bizarre idea. Something fantastical, novel, unique, unthinkable. A violation of common sense or standard thinking.
Notes: In contemporary America, in which novelty is highly valued – perhaps even above elements such as style and workmanship – and things can be “so crazy they just might work,” this might seem to be a phrase of high praise. It is not. (It’s not that bad either; it just may not be as positive as you think it is.)