-[ax], -[ɔx] in the sense ‘mountain-grass, good grass’
The word is not connected with the adjective millteach ‘destructive’ etc. (EG milltech, which goes back to milliḋ ‘spoils, ruins, destroys etc.’ (eDIL˄)).
is derived by MacBain (1896; 1911) from ON melr m. ‘bent-grass’; so also Henderson (1910, 215) and McDonald (2009, 386). However, ON melr nom. would formally yield SG *mealr *[ˈmjɑ̃ɫ̪͡ɑɾ]; ON mel acc. would formally yield *meal *[mjɑ̃ɫ̪].
Final -r of ON melr is the nominative singular ending and not part of the stem.
For millteach, cf. (pl.) millich ‘tufts of good grass’ (Shaw 1780), meillich ‘kind of seaweed’ (CG VI, 107), all of which appear to go back to SG meill, a doublet of meall ‘lump, mass of any matter etc.’ (Dwelly 1911) (cf. O’Rahilly 1942c, 193–95: 194), + the suffix -ach; cf. SG meallag, with the suffix -ag (MacLennan 1925, s.v. mealag: ‘matted roots of grass, of bent’ and, under a separate entry, ‘protuberance, belly’;
Calder (1972, 66) takes SG mealag ‘belly’ to be derived from Eng. belly.
MacDonald 1946, 30, s.v. meallag: ‘a tangled cluster of sea-bent roots found in face of sandbanks; excellent for polishing tin vessels’; Faclan bhon t-Sluagh˄, s.v. mealag, meallag: ‘wiry grass roots found in sandbanks; a scrubber made from these; a tangled mess’, Lewis); mealltrach, with the suffix -ach and interpolated tr (Goodrich-Freer 1897, 67–68, s.v. mealtrach, which she derives from ON melr: ‘grass roots’, South Uist; McDonald 1972, s.v. mealtrach: ‘roots of grass found at the edge of open sandbanks and used for scouring wooden floors with; roots of bent-grass’, South Uist; Faclan bhon t-Sluagh˄, s.v. mealtrach, mealltrach: ‘scrubbing brush made from old marram grass’, Harris, ‘submerged roots which when dry were used for scrubbing chairs’ and ‘the roots of sea-bent, used for scrubbing the miosar [“tub”]’, North Uist, and (s.v. mealtreach [sic]) ‘submerged roots used to make ropes’, South Uist); and mealltrag, with -ag substituted for -ach (ibid., s.v. mealtrag: ‘roots of grass growing in sand – used for scraping pots’, Tiree).
Faclan bhon t-Sluagh˄ also lists meallthìreach ‘submerged roots found in dry peaty soil especially in embankments; these roots were used to make ropes’, Eriskay (where SG *meall-thìreach ‘ground-clump’ appears to be a folk-etymological explanation of mealltrach). The same source lists SG meapaid [mɛ̃pɑdʹ] in the sense ‘scrubber made originally from the roots of sea-bent’, Lewis: meapaid is a loan from MScots moppat, mappat ‘a mop’ (DOST˄).