This invertible root had the basic sense “in(side)” for all of Tolkien’s life. It first appeared as ᴱ√IMI “in, into” and ᴱ√MĪ² in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. imi “in, inside” and ᴱQ. mitta- “enter” (QL/42, 61). Deleted Gnomish forms like G. bi “in” and G. bin “into” might be derived from a strengthened form of the root ✱ᴱ√MBI (GL/21) but Tolkien said the undeleted prefix bi- or G. ba “in“ had a Qenya cognate ve (GL/20), so I think these were based on some other (abandoned) root.
Both ᴹ√MI “inside” and its inversion ᴹ√IMI “in” appeared in The Etymologies of the 1930s (Ety/MI; EtyAC/IMI), though the only derivative of the inversion was ᴹ√IMBE “dell, deep vale” (EtyAC/IMI, IMBE). The root or its inversion appeared a number of times in Tolkien’s later writings as well, up into the 1960s (PE17/41, 92; VT47/11, 30).
This root probably has an extension √MIT as implied by words like mitta “inwards”, mitta- “insert”, and mitya “interior”. Hat-tip to Vyacheslav Stepanov for this suggestion.
This root first appeared as unglossed ᴹ√MEN in The Etymologies of the 1930s, where it seemed to be stationary in nature given its derivatives ᴹQ. men “place, spot” and ᴹQ. ména “region” (Ety/MEN). It was an element in direction words like ᴹQ. formen/N. forven, apparently meaning “✱north place” at this conceptual stage. In Demonstrative, Relative, and Correlative Stems (DRC) from the late 1940s, Tolkien first glossed ᴹ√MEN as “place, spot”, but that instance of the root was revised to ᴹ√NOM (PE23/112 and note #12), and a bit later in the same document Tolkien gave a new meaning to ᴹ√MEN as “purpose, aim” (PE23/112).
In the Quenya Verbal System (QVS) from 1948, Tolkien first gave ᴹ√MEN the gloss “aim at, intend, purpose” with the more specific sense “make for, proceed towards” when used with the allative, but then crossed this out and simply made the gloss “go, proceed” (PE22/103 and note #21). In Common Eldarin: Noun Structure of the early 1950s, Tolkien first gave √MEN the gloss as “place, situation, site”, then revised it to “direction, object, point moved toward” (PE21/79 and note #39).
Thereafter Tolkien mostly gave this root the basic sense “go” (PE17/93, 143, 165; VT39/11; VT42/32). Tolkien gave a somewhat lengthy description of the root in Definitive Linguistic Notes (DLN) from 1959:
> √MEN go, move, proceed in any direction (irrespective of speaker’s position, or assumed point of thought). mēn- a way, a going, a mov[ement] (PE17/165).
Thus in the 1950s and 1960s, Q. formen likely meant “✱north way” rather than “✱north place”. However, in a footnote in The Road Goes Ever On from 1967, Tolkien did say that men had the meaning “direction, region” as part of an explanation of √MENEL “heavens” = men + el (“region of stars”), so it seems Tolkien did not entirely abandon the use of this element as a name for places.
Neo-Eldarin: For purposes of Neo-Quenya, I sometimes see √MEN use for movement in general (as in motion picture) rather than movement in a direction. I personally recommend using ᴱQ. lev- “(intr.) move” from Qenya Wordlists of the 1920s for that purpose (PE16/132) since I think it is important to distinguish “move” from “go”, but if you prefer to use only later words you might be more comfortable expanding the semantic scope of √MEN to movement in general.
@@@ 1950s mendë “will” = “✱intent” and 1964 menta- “have as object, (in)tend”