Why do we say "On a wing and a prayer"?

Meaning:

To do something in hazardous or tenuous circumstances

Background:

This metaphor comes from the image of a damaged airplane attempting to land. It is believed to originate in a 1943 song titled Comin' In on a Wing and a Prayer, written by Harold Adamson (1906-80) with music by Jimmy McHugh. But it was the movie, Wing and a Prayer, which opened in movie theaters the following year, that firmly lodged the expression in people's minds.

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