The noun is derived from Middle English cosin, cosine, cosyn (“blood relative, kinsman or kinswoman; any relative; nephew or niece; first cousin; grandson or granddaughter; descendant; godchild or godparent, or a relative of a godchild or godparent; (figurative) closely related or similar thing”) [and other forms], and then: * from Anglo-Norman cosen, cosin [and other forms], Middle French cosin, and Old French cosin (“collateral male relative more distant than one’s brother; form of address used by a monarch to male monarchs or nobles”) [and other forms] (modern French cousin); and * from Anglo-Norman cosine, Middle French cosine, and Old French cosine (“collateral female relative more distant than one’s sister; form of address used by a monarch to female monarchs or nobles”) [and other forms] (modern French cousine), from Latin cōnsobrīnus (“maternal cousin; first cousin; relation”) (possibly through Vulgar Latin *cōsuīnus, from *cōsobīnus), from con- (prefix denoting a bringing together of several objects) + sobrīnus (“maternal cousin; sister’s son; any nephew”) (from a noun use of Proto-Italic *swezrīnos (“of or belonging to a sister”, adjective) (with the first syllable influenced by Latin soror (“sister”)), ultimately from Proto-Indo-European *swésōr (“sister”), possibly from *swé (“self”) + *h₁ésh₂r̥ (“blood”) (that is, a woman of one’s own blood) or *-sōr (feminine suffix)). The verb is derived from the noun.

chump • chumpion • cousin • cull • cully • dupe • dotterel • easy mark • fall guy • fish • gudgeon • gull • juggins • live one • mark • mug • muggins • pigeon • rube • sap • saphead • schlemiel • sheep • simp • Simple Simon • soft touch • steamer • stiff • stooge • sucker • cousin-german • first cousin • full cousin • kissing cousin • relative