The story
From Hail Mary, a prayer for intercession, from Latin Ave Maria, plus pass (“moving the ball from one player to another”). Attested from the 1970s in the context of American football.
Etymology from Wiktionary, CC BY-SA 4.0.
Hail Mary pass
Today it means: A long forward pass with little chance of completion, typically used by the losing team when time is running out and no other play is practical, in a desperate attempt to score the winning points. Full definition →