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.
This root was the basis for “sword” words throughout Tolkien’s life, but the meaning of the root itself shifted over time. The first appearance of this root was as ᴱ√MAKA in the Qenya Lexicon of the 1910s, unglossed but with derivatives like ᴱQ. mak- “slay”, ᴱQ. makil “sword”, and ᴱQ. makka “slaughter” (QL/57-58). The root was also given the gloss “slay” in a section of the contemporaneous Gnomish Lexicon along with etymologies of names of the various Valar, but this section was deleted (GL/18). Derivatives of this root appeared elsewhere in the Gnomish Lexicon, however, such as G. mactha- “slay, kill” and G. magli “a great sword” (GL/55). Thus, the meaning of this root in the earliest period seems to be “slay”.
The root ᴹ√MAK appeared in The Etymologies of the 1930s, but the entry went through quite a few revisions. The gloss apparently was “cleave” >> “kill, cleave with sword” >> “sword, or verbal [stem] fight with sword, cleave” >> “sword, or as verb-stem: fight (with sword), cleave” (EtyAC/MAK; Ety/MAK). Thus the 1930s root was more directly connected with swords and wielding swords, and its derivatives included ᴹQ. makil/N. magol “sword” and ᴹQ. mahta-/N. maetha- “fight” (Ety/MAK).
In Tolkien’s later writings, the words for “sword” remained nearly the same: Q. macil and S. megil (PE17/130, 147), but the gloss of the root √MAK varied considerably based on whatever linguistic puzzle Tolkien was trying to solve at that particular moment. In notes associated with the Quendi and Eldar essay from 1959-60, its gloss was very similar to that from The Etymologies: “cut, hew with a sharp edge” (VT39/11). In notes associated with The Shibboleth of Fëanor from 1968 Tolkien glossed {m(b)aka- >>} maka- as “forge metal” as part of a new etymology of the name S. Maglor as an adaptation Q. Makalaure “Forging Gold” (PM/353; VT41/10).
In notes on Eldarin Hands, Fingers and Numerals from the late 1960s Tolkien gave √MAK “strike” as the basis for ✶makwā > S. mâb “hand”, but this idea was rejected immediately (VT47/19). The same gloss “strike” appeared in a document on the Common Eldarin Article, deleted there with a reference back to the “Hands, Fingers and Numerals” document (PE23/144). This was part of Tolkien’s rather surprising decision to abandon the long-standing root √MAP (VT47/20 note #13); elsewhere in these notes he said √MAK meant “kill, slay” as it did in the 1910s (VT47/20).
Lokyt suggested in a Discord chat from 2018 that there may be a common underlying meaning for all these glosses, referring to “the movement one does when chopping with a tool”, as supported by the late 1960s gloss “strike”. Assuming this is true, the other associations of the root (“sword; slay; fight; forge”) may be the result of a narrowing of the meaning of the root in more specific contexts. I assume the based meaning of this root is something like “strike” purposes of Neo-Eldarin, as it allows us to retain the largest set of derivatives of the root.