It is a very long pass made in football, which has lesser chances of success but such a shot is made in the case where the time remaining is lesser to aim a shot with normal passes. In general, terms, an effort made with a little hope of success.
Diagrams
Originated from/Coined by
Further Reading/ References
https://en.wikipedia.org/wiki/Hail_Mary_pass
https://www.ldoceonline.com/dictionary/hail-mary-pass