The term Pongo comes from the days when soldiers were stationed on board ships to protect the Navy when sailing abroad. Usually the first to be sent ashore when the ship docked, soldiers would carry out all sorts of different tasks.
One important (the most important... surely) task being the setting up of a brewery. The main part of the still being called a pongo. Hence the nick-name giving to the soldiers who would be sent to do the job "send the pongos ashore". The name seems to have filtered down through the years and is used today by the Navy towards members of the Army.