Origins
The word 'class' entered English in the 1570s directly from Latin 'classis,' a word with a long and politically charged history. In its earliest attested use, 'classis' referred to the divisions of the Roman citizen body established, according to tradition, by the sixth king of Rome, Servius Tullius, in the sixth century BCE. Servius divided all Roman citizens into six groups based on wealth, primarily for the purposes of military service and taxation. The wealthiest — those who could afford to equip themselves with full armor — formed the first 'classis'; the poorest, who could contribute little to the army, were 'infra classem' (below the class system entirely). The Roman 'classis' was thus, from its inception, a system of ranking people by economic power.
The word itself appears to derive from the archaic Latin verb 'calāre' (to call, to summon, to proclaim), from PIE *kelh₁- (to call, to shout). The 'classis' was originally 'a summoning' — the calling together of citizens for military muster or census. The same root 'calāre' produced Latin 'calendae' (the Calends, the first day of the Roman month, when debts were called in and the new month was publicly proclaimed), giving English 'calendar.'
The derivative 'classicus' had an enormous impact on European culture. In its original sense, 'classicus' meant 'belonging to the first class' — a citizen of the highest wealth rank. The second-century grammarian Aulus Gellius used 'classicus scriptor' (a first-class writer) to mean an author of the highest rank, as opposed to a 'proletarius scriptor' (a proletarian writer, one from the lowest class). This metaphorical use — transferring the language of social hierarchy to literary quality — gave rise to 'classic' and 'classical,' words that now denote cultural excellence. When we call a work 'a classic,' we are still, two millennia later, placing it in the first rank of Servius Tullius's census.
Latin Roots
Latin 'classis' also developed a military meaning: a fleet of ships. The connection to the original sense is straightforward — a 'classis' of ships was a group called together for naval service, just as a 'classis' of citizens was a group called together for military assessment. This naval sense survives in English only indirectly, in specialized historical usage.
The educational sense — a group of students taught together — emerged in the sixteenth century, when European schools and universities began organizing students into ranked groups based on ability or progression. The Jesuits, who established a highly influential model of schooling in the sixteenth and seventeenth centuries, were among the first to use 'classis' systematically for groups of students advancing through a curriculum in stages. From this usage, 'class' came to mean both the group of students and the session of instruction itself.
The social sense — 'class' as a division of society — entered English political discourse in the seventeenth and eighteenth centuries and became central to social thought with the rise of industrialism. Karl Marx's class theory, Adam Smith's analysis of social orders, and the entire tradition of class-based social analysis rely on a word that still carries the DNA of Servius Tullius's wealth-based census.
Modern Legacy
The word's journey from 'a summoning' to 'a wealth rank' to 'a group of students' to 'a social stratum' illustrates how a concrete administrative act — calling citizens together to count them — can generate layer after layer of abstract meaning. A modern sentence like 'She is taking a class on class structure in classical civilizations' uses three derivatives of 'classis,' each in a different sense, all traceable to the same Roman census.