This root was intertwined with the root for “hand” words: √MAH or √MAƷ. Indeed, in The Etymologies of the 1930s the roots ᴹ√MAG and ᴹ√MAƷ were variants of one another, with ᴹ√MAƷ glossed “hand” and ᴹ√MAG glossed “use, handle”, the latter with the derivatives like ᴹQ. mára/N. maer “useful, fit, good (of things)” and N. maen “skilled, clever” (Ety/MAƷ|MAG).
In later writings, however, the sense of this root shifted more towards “good”. In etymological notes from the late 1950s Tolkien still gave both √MAG or √MAƷ as the basis for hand words (PE17/161-2), but in Definitive Linguistic Notes (DLN) from 1959, Tolkien gave √MAGA the gloss “good (physically)” (PE17/149) or “to thrive, be in a good state” (PE17/162). In notes on Eldarin Hands, Fingers and Numerals from the late 1960s, Tolkien said the root for “hand” was √MAƷA, and there was a distinct (but possibly related) root √MAG, described as follows:
> MAGA, a stem meaning “good” — but without moral reference, except by implication: sc. it was not the opposite of “evil, wicked” but of “bad (damaged, imperfect, unfit, useless)”, and the adjectival stem derived, *magrā, meant “good for a purpose or function, as required or desired, useful, proper, fit” (VT47/6).
A similar distinction between √MAƷA “hand” and √MAGA “good, useful” appeared in 1968 notes (VT47/19). In Late Notes on Verb Structure (LVS) from 1969, Tolkien gave √MAGA the gloss “have, possess” to serve as the basis for the irregular verb Q. mai-, but this note was crossed through and I suspect it was a transient idea (PE22/148 and note #24). Thus, “good, useful” seems to be the best interpretation for the root √MAG.