This root appears the Etymologies in the extended form ᴹ√KUNDU, from which ᴹQ. †kundu, N. †cunn “prince” and N. Felagund “Lord of Caves” were derived. In later writings (PE17/113, 117), Tolkien explored the possibility of connecting Felagund to the names of Fingon and Turgon, deriving all three from a primitive form ✶kondō (<< ✶kundō) and using a new form of this root: √KON (<< √KUN). He even went so far as to change the first of these names to Felegond with an “o” (PE17/118).
However, in still later writings (PM/345), Tolkien established that the element -gon “lord” in S. Fingon and Turgon was ultimately derived from Q. cáno “commander”. In the same text, he revised the etymology of S. Felagund so that it was derived from a Khuzdul name Kh. Felakgundu “Cave-hewer” (PM/352), as described in The Silmarillion index (SI/Felagund). These revisions make it likely that Tolkien abandoned the form √KON.
The later status of earlier √KUN is unclear, however. √KUN might have been rejected along with √KON, or Tolkien may have restored it. As evidence of its restoration, †cund remained as an element in two older (Sindarin?) names: Baragund and Belegund. Furthermore, its Quenya derivative Q. cundo appears as an element in the later name Q. Carma-cundo, albiet with a new gloss “guardian”.
If we accept the restoration of √KUN, its Quenya derivative cundo “prince, lord, guardian” could be considered valid, possibly along with the verb cunya- “to rule”. Its Sindarin derivative †cund “prince” should probably be considered archaic, however, replaced by the time of the Third Age with caun¹.