Definitions for hail mary
Random House Webster's College Dictionary
Ref: Ave Maria.
Origin of Hail Mary:
1300–50; ME, trans. of ML Ave Maria
Ave Maria, Hail Mary(noun)
a salutation to the Virgin Mary now used in prayers to her
A prayer calling for the intercession of the Holy Mother, for Catholics an essential element of the rosary, and with use in certain other denominations.
A long forward pass with little chance of completion, typically used when time is running out and no other play is practical.
An act done in desperation, with only a very small chance of success.