Origin of Phrases - O
Over the top
Meaning: Too much, overdone, excessive.
Example: Wearing that bikini is one thing, but a bikini with a mink coat puts you over the top.
Origin: During the first world war a charge over the protective battery which ran alongside a trench was called "going over the top." Such a charge usually resulted in many casualties, as did most operations during that most tragic conflict.
Since the casualty rate was very high, it took remarkable bravery to go "over the top". Some considered it excessively brave and the phrase has come to be associated with excess.
Specifically the phrase means that disclosing a military secret to the enemy could result in large loses, such as the sinking of an entire ship.