RAF RN slang. Affectionate nickname for Her Majesty's Army. Abbreviated from the phrase; "Where the Army goes, the pong goes". A slightly unkind reference to the personal hygiene arrangements of a fine body of men and women whilst on manoeuvres.
Also Pongo: Latin word for ape.
The term may be a relic of the Army's service in the Peninsular war from 1807-14. The Portuguese word for Bread is "pão", pronounced "Pong." A term unfamiliar to Matelots, who ate "hard tack" and "soft tack".