The third-age Sindarin word for “prince” (PE17/102), appearing in its plural form conin in the Praises of Cormallen: Daur a Berhael, Conin en Annûn “Frodo and Sam, princes of the west” (LotR/953; Let/448). More generally, caun means “chief” or “head”, and appears as an element in the word condir “mayor, (lit.) chief-man” (SD/129). There is another more archaic Sindarin word for “prince”, †cund, which appears in some old names from Beleriand: Baragund, Belegund and (possibly) Felagund*.
Possible Etymology: The etymology of caun “prince” is unclear. David Salo suggested (GS/245) that it is derived from ✶kānō “leader, commander” (originally “crier, herald”). This primitive word also appears as an element in the Quenya names of the sons of Fingolfin: Q. Findecáno (S. Fingon) and Q. Turucáno (S. Turgon). However, caun might instead be derived from an a-fortified form of the root √KUN(DU) “lord; to lead”, so that: ✶kun- > ✶kaun- > S. caun. This second derivation would make S. caun a cognate of Q. cundo “prince”.
Given the uncertain status of √KUN in later writings, a derivation from ✶kānō might be preferable. However, Tolkien stated than in Sindarin, the derivatives of √KAN were used for “cry out, shout, call” but not “order, command” (PM/361). If the Sindarin derivatives of √KAN had nothing to do with leadership, perhaps the archaic word †cund “prince” was altered to caun under the influence of Q. cáno and names like S. Fingon and Turgon, and that is the origin of the modern Sindarin word.
Grammar: This word has an irregular plural: conin “princes” (LotR/953; Let/448). For other words such as êl “star”, such a plural indicates the preservation of ancient priminite n, lost at the end of the singular form. This seems unlikely to be the case here, so likely this irregular plural is by analogy with other plural words.
pl1. cónin {ō} n. prince, chief, head.