Just like the phrase “On a wing and a prayer” this phrase is also from World War I.
Back then, a charge over the protective battery which ran alongside a trench was called “going over the top.”
Because the casualty rate was very high in such instances, one had to be quite brave to go “over the top”.
Some considered it to be excessively so and soon this phrase came to be associated with being too much and excessive.
What began as a World War I buzz phrase about bravery has, sad to say, become a very negative comment regarding one’s effort or plan to achieve a goal.
Historically Speaking 