Origins
The word 'cousin' is an etymological fossil of a kinship system far more precise than the one English uses today. Latin had a remarkably fine-grained vocabulary for cousins: 'consobrinus' (children of two sisters — maternal first cousins), 'patruelis' (children of two brothers — paternal first cousins), 'amitinus' (child of a paternal aunt), and 'avunculi filius' (child of a maternal uncle). Each term encoded not just the generational relationship but which combination of siblings had produced the children. Old French collapsed all of these into a single word 'cosin,' and Middle English borrowed that simplified term wholesale.
The Latin 'consobrinus' is built from two elements: the prefix 'con-' (from 'cum,' meaning 'together' or 'jointly') and 'sobrinus,' a word derived from 'soror,' the Latin word for sister. 'Soror' descends from PIE *swésor, one of the most securely reconstructed kinship terms in the proto-language, attested in Sanskrit 'svásar-,' Old English 'sweostor' (modern 'sister'), Greek 'eor' (in compounds), Gothic 'swistar,' and Armenian 'k'uyr.' The PIE root may contain the element *swe- (one's own, self), giving *swésor a sense of 'one's own woman' — a member of one's own household group before marriage.
The compound 'consobrinus' therefore means literally something like 'those who share a sister' — the children of women who are sisters to each other. It is a fundamentally matrilateral concept, organised around the female sibling link. This reveals something of the social logic behind the Latin kinship terminology: the maternal cousins (consobrini) were felt to be more intimately related than the paternal ones, reflecting perhaps the closer emotional ties characteristic of the maternal line in Roman and earlier IE society.
French Influence
When Old French generalised 'cosin' to cover all cousins, a significant terminological complexity was lost. Modern Romance languages mostly follow this flattened path: French 'cousin,' Italian 'cugino,' Portuguese 'primo' (which instead reached back to Latin 'primus,' first, as in 'primo hermano,' first sibling-like relation). Spanish 'primo' is particularly revealing — by calling a cousin a 'first' (as in first degree of collateral relation), Spanish encodes exactly the information that 'consobrinus' once encoded with different vocabulary.
In medieval England, 'cousin' was used with extraordinary latitude. The legal and diplomatic registers of the Middle Ages used 'cousin' for any relative not in the direct line — it could mean a first cousin, a second cousin, a nephew, or simply a fellow nobleman of allied family. The Paston Letters (15th century) show the word applied to figures who were clearly not first cousins in the modern sense. Monarchs regularly addressed each other as 'cousin' in state correspondence as a mark of royal solidarity — a usage that was formally polite fiction rather than genealogical claim, though often supported by the dense intermarriage networks of European royal houses.
The English pronunciation 'cuzzen' reflects the regular reduction of the unstressed final syllable, a process that happened in many French loanwords as they naturalised into English phonology. The spelling 'cousin' preserves the French form more faithfully than the pronunciation does, a common pattern in English orthography where the spelling was fixed at an earlier stage than the spoken form.
Proto-Indo-European Roots
The connection between 'cousin' and 'sorority' (from Latin 'soror') is etymological though not visually obvious: both words ultimately trace back to PIE *swésor. 'Sister' itself, via Old English, is a third reflex of the same root in English — meaning that 'sister,' 'sorority,' and the buried 'soror-' inside 'cousin' all go back to the same Proto-Indo-European word, three branches of a single ancient kinship term that have been living unrecognised in English all along.