A verb meaning “to slay” derived from the root √NDAK, best known from its passive participle dangen as in Haudh-en-Ndengin “Hill of Slain” (S/197). Tolkien wrote a set of possible past forms aðag, aðanc, aðarch in notes from 1962 (PE17/131), and the verb appeared in its Noldorin-style infinitive form degi “to slay” in The Etymologies of the 1930s under the root ᴹ√NDAK “slay” (Ety/NDAK), along with another couple of (Noldorin) past forms: danc, degant (EtyAC/NDAK). The verb form ᴱN. (n)dag- “to slay” appeared in Early Noldorin Word-lists of the 1920s (PE13/141), but its present form dág was glossed “kills” and in the contemporaneous Early Noldorin Grammar the form dagion was likewise glossed “I kill” (PE13/130). Thus “slay” and “kill” are both viable translations.
Possible Etymology: In notes from around 1962, Tolkien gave ✶dankĭnā as the primitive form of its passive participle dangen, indicating a root √DAK rather than √NDAK, which is also consistent with its nasal mutated plural form on that page: {i dengin >>} i nengin (PE17/133). The 1964 past forms aðag and aðanc also seem to indicate derivation from √DAK (PE17/131). In notes from around 1967, however, Tolkien had the mixed mutated form n(d)engin in the phrase i·m(b)air en N(d)engin, indicating √NDAK, and he consistently gave nac- for the equivalent Quenya forms, so the early 1960s flirtation with √DAK seems to have been a transient idea.
Neo-Sindarin: For purposes of Neo-Sindarin, I would assume the root is √NDAK, and hence I’d give it the past form ✱annanc “slayed” rather than aðanc.
A verb in The Etymologies of the 1930s appearing as {bactha- >>} banga- “trade” under the root ᴹ√MBAKH “exchange” (Ety/MBAKH).
Conceptual Development: The Gnomish Lexicon of the 1910s had G. bag- “sell, trade” that Tolkien indicated was originally related to G. bad- “travel” (GL/21). Early Noldorin Word-lists of the 1920s had ᴱN. bag- {“sell” >>} “to sell, trade” (PE13/138).