This word is only directly attested in The Etymologies, appearing as N. †cunn “prince” (Ety/KUNDŪ) and marked as an archaic form (EtyAC/KUNDŪ). It still appears in later writings as a element in some first-age Sindarin names from The Silmarillion: Baragund, Belegund and perhaps Felagund (see below). Its Quenya cognate Q. cundo also appears in later writings (PM/260, PE17/117-8), indicating that †cund may have remained conceptually valid.
Alternate etymologies of the name Felagund complicate this picture, however. In a 1959 etymology Tolkien said Felagund was derived from Dwarvish Felakgundu “Cave Hewer” (PM/352), and in a 1969 etymology Tolkien said it meant “den-dweller” and that “the ending -gund could not be interpreted from Eldarin” (NM/304). This indicates cund “prince” may have been abandoned, and that Baragund and Belegund were either (a) remnants of earlier conceptions or (b) reinterpreted as Mannish (Beorian) names.
Elsewhere, the third-age Sindarin word for “prince” is said to be caun (PE17/102), so even if survived conceptually, it seems likely the older form †cund fell out use.
pl1. cónin {ō} n. prince, chief, head.