In the old Navy, an officer's commission was hand-written on heavy parchment. According to some sources, the newly commissioned or promoted officer held a dinner for his shipmates and friends. During the course of the evening, the new commission was rolled into a cone, the small end folded up to form a cup. This paper cup was passed around the table for all the guests to toast the new officer. Thus, the new commission was "wetted down." Considering the importance of the document, however, this interpretation may be doubtful. Commissions inmost of the navies were signed and issued by the President and were of great legal and personal value.
According to other sources, the wetting down party was once quite a rough and tumble affair. It was the custom for the officer to wear his new uniform or stripes for the first time at the wetting down. The guests would then proceed to christen the uniform, the occupant, and the commission with whatever liquid refreshment (paid for by the victim) was available. Over the years, however, Navy life has became more calm, the price of gold braid has skyrocketed and a literal christening is not usually condoned. It might even be considered downright unsociable.
