1
general::
A remote possibility of success, as in It's a long shot that Joan will actually finish the marathon, or He may be a good programmer, but he's a long shot for that job. This expression alludes to the inaccuracy of early firearms, which when shot over a distance rarely hit the target. It is commonly used in horseracing for a bet made at great odds. A related phrase is not by a long shot, meaning ‘‘not even remotely," as in I'll never make it to California in three days, not by a long shot. [Late 1800s]
American Heritage Idioms