Hail Mary

Short definition from Wiktionary

Noun

1. 1
   A prayer, of established or conventional wording, calling for the intercession of the Virgin Mary.
2. 2
   A risky last-ditch effort with great benefit but little chance of success; one whose success would require divine intervention; a Hail Mary pass.

Verb

1. 1
   To pray by saying a Hail Mary.