An adjective in notes from 1967 derived from primitive ✶ndakna (PE17/68) and hence based on the root √NDAK “hew, slay”, so perhaps meaning “✱hewn” or “✱slain”. For purposes of Neo-Quenya, I would assume “✱hewn”, since nahtana likely means “✱slain”.
Quenya
nanca
slain
nanca
adjective. *hewn, slain
-na
suffix. slain
A shorter ending -na also occurs, e.g. nahtana "slain" (VT49:24); the example hastaina "marred" would suggest that *nahtaina is equally possible. In the example aistana "blessed" (VT43:30), -na may be preferred to -ina for euphonic reasons, to avoid creating a second diphthong ai where one already occurs in the previous syllable (*aistaina). In PE17:68, the ending -ina is said to be "aorist" (unmarked as regards time and aspect); the same source states that the shorter ending -na is "no longer part of verbal conjugation", though it obviously survives in many words that are maybe now to be considered independent adjectives. See -na #4.
nanca adj. *"slain" (PE17:68); see -na