v1.0
Published 01/10/24
bac m. [b̥aʰk], cf. (Lewis) /bɑk/ (Oftedal 1956, 103, 110), gen. bac, baca (occasionally baic 
?Under the influence of the homonym bac, see below.
). In the sense ‘peat-bank’ (Mackay 1897, 94) or ‘(sand-)bank’ (Oftedal ibid., 335), this word is derived from ON bakki 
Oftedal ibid., 105, 110; Stewart 2004, 408; McDonald 2009, 339; 2015, 155; Ó Muirithe 2010. Mackay (ibid.) cites Ice. bakki.
m. ‘bank, slope; hill’, but the etymon must have been bakka, obl. case of the weak noun bakki, because of the non-palatal final consonant in Gaelic. As the case stands, then, we must assume a development ON bakka > SG *baca > bac with apocope, and this is the argument put forward by Cox (2022, 468–70) with regard to the generic in the Lewis village name Am Bac ‘the slope’, gen. a’ Bhac.
Cf. the plural forms in the place-names Na Bacannan and Na Bacaidhean (ibid.). NB The presence of reflexes of ON bakki in Old Norse loan-names in place-names such as SG (Lewis) Bacabhat (Cox 1994, 31) and SG (Sutherland) Callbacaidh (Fraser 1979, 22) is not germane here: they do not constitute Old Norse loan-words in Gaelic (pace McDonald 2009, 339).
However, Cox’s alternative suggestion that SG bac derives from a monosyllabic *bakk, accusative of an otherwise unattested strong noun ON *bakkr, is also made by Borgstrøm (1941, 101: ‘bakki (bakkr)’).
?Cf. Scots bakk ‘bank, slope; edge, bank’, now commonly ‘peat-bank’ < ON bakki (Jakobsen 1928).
A ‘peat-bank’ is more specifically SG bac-mòine (Henderson 1910, 118; MacBain 1911; Borgstrøm ibid.) or bac-mòna (MacLennan 1925, s.v. bac) etc., an open compound with one form or another of the genitive of SG mòine f. ‘peat’ used adjectivally.
The compound was recorded as the equivalent of poll-mònadh ‘peat-bank’ in South Uist (Faclan bhon t-Sluagh).
The open compound bodach bac or bodach a’ bhaic refers to ‘the first cut or outside layer of a peat-bank’ in Sutherland and Wester Ross (Faclan bhon t-Sluagh; Wentworth 2003 s.v. peat-bank), while the similarly constructed gàrradh-bac in Coigach (ibid.) is the equivalent of gàrradh dubh ‘dyke or wall built with peat’ elsewhere.
Note that EG bacc ‘angle, bend, corner; hooked implement; hindrance’ (eDIL˄) also yields SG bac. In the sense ‘angle’, native bac survives for example as the term for an ‘(L-shaped) earmark’, ‘the hollow of the bent knee’ (e.g. bac air bhac ‘with legs crossed at the knees’ (McDonald 1972, 34)) and in the open compound bun-bac m., 
With SG bun m. ‘the base, bottom or foot of something’. AFB˄ cites bun-bac and the alternative bun a’ bhaca.
referring to ‘the eaves of a house, the lower part of a roof just above the wall, the gap between the wall of a house and the thatched overlap, the point where the rafters come to rest on the top of a wall’ (Faclan bhon t-Sluagh). Extended senses of bun-bac recorded in the same source include (Eriskay) ‘horizon’ 
See also McRury 1889, 111. McDonald (1972, App. I, p. 257) records the plural form bunacha-bac, which is said to be ‘some unknown place outside Uist’ but later (ibid., Suppl., p. 306) it is given as ‘horizon’ (see Matheson 1960, 210)
; (Scalpay) ‘lug-sail (a fore-and-aft, four-cornered sail)’, which may be the result of confusion with and an extension of the meaning of bac (air a’ chrann); 
The expression an seòl bac air a’ chrann is used to describe sailing a boat with the wind side-on (as opposed to sailing into the wind or with the wind behind), see Faclan bhon t-Sluagh, s.v. seòl bac + illustration.
and (Scalpay) ‘lying at anchor’, from the expression na ràimh ri bun-bac (ibid.), literally ‘the oars at bun-bac’, i.e. ‘pointed to the horizon and therefore not in use’. EG bacc in the sense ‘hooked implement’ survives for example in the terms bac ‘rowlock or thole-pin’, aparan bac ‘a plate protecting the area under the oar, between the thole-pins’ (ibid.
Dwelly 1911, s.v. bàta 39a, gives ‘apran, bac’.
) and presumably in the expression gu bac ‘to the full’ (MacLennan 1925, s.v. bac), literally ‘up to (the) bac’, cf. Eng. up to the gunwales. EG bacc in the sense ‘hindrance’ survives for example in the commonly used noun bac ‘impediment, obstruction’ and the verb bac and verbal-noun bacadh.
Ir. (Donegal) bachta m. ‘turf-bank’ (Quiggin 1906, 114; Wagner 1979, 186) appears to be a loan from SG bac in the (more southerly) form [b̥axk] (rather than [b̥aʰk]), interpreted as *bachd, hence Ir. bachta, cf. SG seachd -[xk] but Ir. seacht -[xt] ‘seven’ (EG secht; for the development of EG -cht in Scottish Gaelic, see O’Rahilly 1976, 150). For final -a, cf. Ir. cuideachta but SG cuideachd ‘company’.