Shellbacking is the process by which a "Wog" becomes a "Shellback." An explanation.
When a recruit joins the Navy, they are a wog. They are not often referred to as such. They remain as such until during a cruise they cross the equator. Then they are initiated, and become a Shellback. The initiations vary from ship to ship, but there are certain common threads. One, the 'wog' is physically tormented in various ways during the course of the initiation. Two, the 'wog' never walks upright. Three, one must declare before King Neptune whether he is 'wog' or 'Shellback.' (Of course I won't be saying which is the proper answer, so as not to spoil it for any potential Sailors and Squids out there.) After the trials, which range from torturous to silly but torturous, the new Shellbacks are granted membership in the Shellback club.
On a sad note, this 'trial by fire' is fading quickly due to the presence of females onboard naval vessels. They tend to object to being flogged by lengths of fire hose. Although all male ships still unofficially perform the rite.
Once you are a shellback, you are always a shellback.