Answer:
plebs is the roman word for the lowest class of roman citizen
the class was the called plebeian class, and individuals were referred to as plebs. (basically the working class)
the term plebs was introduced to the American army by the British Army, as plebs was a term used often for new boys in British public schools, (who taught Latin and classical history to its boys) (the term then carried over in to the army when many of these boys became officers), Because of the close ties between the two nations armies many British customs and sayings passed into general usage within the US military (officers mess, for example)
a person being called a pleb means they are at the bottom of the ladder, military academies and sororities are hierarchical in nature and as such new people start at the bottom and need to work their way up the ladder, hence they start at the pleb level