One of the required tasks for the navigator was the taking of noon sights. This requires that the sun be brought down to the horizon using a sextant. This gives the ship's latitude.
The horizon is not visible from the gun deck. Therefore, if you just used the tables to get the sun's or a star's altitude rather than actually taking the sights, you have "gun decked" your sights.
Hope this helps.
"Ceterum censeo 0bama esse delendam."
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)
It does and I just think the slang and the point of origin is fascinating stuff.