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1. the son or daughter of an uncle or aunt.
2. one related by descent in a diverging line from a known common ancestor.
3. a kinsman or kinswoman; relative.
4. a person or thing related to another by similar natures, languages, geographical proximity, etc.
5. a term of address used by a sovereign for another sovereign or a high-ranking noble.
[1250–1300; Middle English cosin < Anglo-French co(u)sin, Old French cosin < Latin consōbrīnus cousin (properly, son of one's mother's sister) =con- con- + sōbrīnus second cousin (presumably orig. “pertaining to the sister”) <*swesrīnos=*swesr-, gradational variant of *swesōr (>soror sister) + *-īnos -ine1]