This root appeared in a late etymology of the name S. Maeglin (“Sharp Glance”) as √MIK “pierce”, along with several other derivatives including the word Q. hendumaika “sharp-eye[d]” (WJ/337). In earlier versions of the tales, the name was given as G. Meglin, but this form of the name was not given a derivation until the 1930s, when N. meglin appeared in The Etymologies as an adjectival form of N. megli “bear” (Ety/LIS). The form of this name became Maeglin in Silmarillion drafts from the 1950s and 60s (WJ/122 note §119), though when Tolkien devised its new derivation isn’t clear. The (unglossed) Quenya word Q. maica appeared in the second version Tengwesta Qenderinwa (TQ2) from around 1950, and it may be related to √MIK “pierce”, but without a translation it is hard to be sure.
Primitive elvish
ter
root. pierce
mik
root. pierce
ter
root. pierce
This root first appeared as unglossed ᴱ√TEŘE [TEÐE] in the Qenya Lexicon of the 1910s, with derivatives like ᴱQ. teret “auger, borer, gimlet”, ᴱQ. tereva “piercing, acute, shrill, sharp”, and ᴱQ. teste “worm”; another set of derivatives based on Q. teren “lissom, lithe” were marked by Tolkien with a “?” (QL/91). In the Poetic and Mythological Words of Eldarissa the root teře was glossed “pierce” (PME/91). Possibly related forms in the contemporaneous Gnomish Lexicon include G. tertha- “devour, destroy” and G. tereg “worm”, but if so they may represent a shift of the root from ᴱ√TEÐE >> ✱ᴱ√TERE (GL/70).
Indeed, in The Etymologies of the 1930s the root was given as ᴹ√TER “pierce” with derivatives like ᴹQ. ter/N. trî “through” and ᴹQ. tereva/N. trîw “fine, acute, [N.] very slender” (Ety/TER). As originally written, the root was ᴹ√TERÉW, but Tolkien changed this to ᴹ√TER and added an extended form ᴹ√TERES that was the basis for ᴹQ. terra/N. tess “fine pierced hole” (EtyAC/TER). The root √TER “pierce” appeared again in notes from 1957 on the origin of Q. Vairë “Weaver” as a variant of √THER “sew” (PE17/33). This specific note was marked through, but given Tolkien’s ongoing use of Q. ter “though”, the root √TER probably survived.