The word 'proletariat' is used here and hereafter in this Study to mean any social element or group which in some way is 'in' but not 'of' any given society at any given stage of such society's history. That is, it is used in the scenes of the Latin word proletarius from which it is derived. In Roman legal terminology, proletarii were citizens who had no entry against their names in the census except their progeny (proles). <...> To say that 'proletarians' contribute nothing to the community but their progeny is a euphemism for saying that the community gives them no remuneration for any other contributions that they may make (whether voluntarily of under compulsion) to the common weal. In other words, a 'proletariat' is an element of group in a community which has no 'stake' in that community beyond the fact of its physical existence. (c) A. Toynbee 'Study of History'

Иными словами, в Древнем Риме словом "пролетариат" называли людей, напротив имен которых во время переписи не было написано ничего, кроме имён их детей. То есть, "пролетариат" - это элемент социальной группы, который кровно заинтересован в обществе только фактом своего физического существования.