Publishing history:v1.0
v1.0: 07/04/26
àirc f. [aːɾ̥ʲkʲ], 
Cf. [âėrk] (McAlpine 1832, s.v. airc: Islay), [aːrˈcg] (Dieckhoff 1932, s.v. airc), /aːrʲçgʲ/ (AFB˄).
McDonald (2009, 430) opines that it is ‘[d]ifficult to differentiate between the two possible sources [i.e. Old Norse and Latin]’.
Shaw (1780) cross-references arc to airc, Armstrong (1825) †arc to airc, and HSS (1828) àrc to àirc.
Cf. Plunkett 1662, f.30v, s.v. arca: áirc (for the variant árg, cf. arg (eDIL˄)); Lhuyd 1707: airc ‘ark’; O’Brien 1768: airc ‘ark’, Lat. arca; O’Reilly 1817; 1864: airc ‘chest’; but note the extended sense ‘body’ in O’Clery’s (1643) arc .i. corp.
SG àirc includes the senses ‘chest, box, ark’ and is used biblically of both Noah’s Ark and the Ark of the Covenant. The senses ‘large granary’ (Shaw 1780; HSS 1828) and ‘granary’ (Armstrong 1825; Dwelly 1911) probably mean ‘meal bin’, cf. Scots ark in the sense ‘large chest for storing corn, meal, fruit etc.’ (SND˄, s.v. 1ark) and Ulster-Scots ark, airk, erk ‘meal bin’ (Macafee 1996). Faclan bhon t-Sluagh˄ also lists àirc in the senses ‘hake for fodder’ 
Cf. SND˄, s.v. [Scots] heck ‘a rack or slatted wooden (or iron) framework’ etc., specifically ‘a rack of wooden spars for holding fodder, fixed to the wall of a byre or stable or secured to a portable framework for use when feeding sheep, etc. in the open’.
≈‘The word is probably derived from Lat. arcus “a bow”, although its later application might suggest Lat. arca “chest or box” ’ (ibid.).
Note that AFB’s entry (àrc) includes the senses ‘cork; bung, stopper; bottle top, cap; bracket/tree fungus’, but no doubt these have a different origin: for SG àrc (also àirc, e.g. Dieckhoff 1932, s.v. airc: Glengarry, and Faclan bhon t-Sluagh˄: Skye), àrca (HSS 1828; Dwelly 1911) and dim. àrcan (e.g. Lhuyd 1700, 185 XXIV.33: cork ‘ârkan’; Dwelly 1911), cf. Mx eirkey (Kneen 1978) and eairkey (Kelly 1991) ‘cork’, and Ir. arcan ‘cork’ (Lhuyd 1707), farcan ‘fungus’ also ‘oak’, farcán ‘cork, plug, stopper, dim. of farc and arc’ (eDIL˄). (In Ó Dónaill 1977, Ir. farcán appears to be subsumed under Ir. fárcán, fadharcán ‘knot (in timber) etc.’ (cf. eDIL˄, s.v. fadarcán: ?based on EG aḋarc ‘horn’).)
Carmichael continues, ≈‘a waterfall on the south shoulder of Cruachan is called Arc Eas Bheann ‘the àrc of the waterfall of the peaks’, to which his editor Angus Matheson adds ‘àrc “vulva vaccinea”, Dwelly’ (see above). Ben Cruachan (NN069304) is in Scottish Gaelic traditionally Cruachan Beann (e.g. George Campbell Hay (1915–1984), in Byrne 2000 I, 82; for further references, see Ainmean-Àite na h-Alba, s.v. Ben Cruachan) ‘the mountain of peaks’, with (unlenited) genitive plural of SG beinn ‘mountain’ (which in this name normally bears the primary stress, as in the song ‘Cruachan Beann’ (Tobar an Dualchais, ID 12549) by Pàdraig Mac an t-Saoir (1782–1855) of Leitir Beann (cf. OS 1848–83 Leitir NN105261), pace Watson and Murray (2015, 174), who place primary stress on the first element), although a modern rendering is Cruachan Bheann (e.g. Taylor 2011, 181, s.v. Ben Cruachan), with lenition of an indefinite genitive plural noun according to modern practice. Carmichael’s Arc Eas Bheann denotes The Falls of Cruachan NN079269, and was recorded by William J. Watson (Carmichael-Watson: CW67, p. 60r) as Eas arc beann [‘the stream of the water passage—’] from ‘Charles MacDonald (near 80) Barracheandar, Taynuilt’ in 1914, but as Eas dearc beann [with SG dearc ‘opening’ (EG derc)] from the Rev. M. N. Munro (probably the Rev. Malcolm Nicolson Munro, b. Uig, Lewis 1869, d. Taynuilt 1934; for an obituary, see An Gaidheal XXIX, Am Màrt 1934, 85), but earlier by Archibald Smith (1867, 224–25) as Easdurchabeann [with SG dorcha ‘dark’]. Evidently, the name has been subject to change. One possible scenario is that a place-name with eas ‘waterfall’ as generic was created using an existing name (i.e. *Dearc Beann ‘the opening—) as specific. However, through folk etymology *Dearc Beann was reinterpreted as *Àrc Beann, using the Scots loan-word àrc to highlight the feature originally denoted by dearc, but which was now confused with the homophonous, but semantically implausible, dearc ‘berry’. Carmichael’s Arc Eas Bheann may represent a ‘corrected’ version of the later form of the name. Smith’s Easdurchabeann assumes a name-form *Eas Dorcha Beann ‘the dark waterfall—’, which may have been created quite independently of *Eas Dearc Beann, two different names for the same feature not being out of the question. The final, stressed element in these forms, as well as in the name Leitir Beann, may be a truncated form of Cruachan Beann.
(Note that, besides commenting that Cruachan Beann, if it means ‘[the] haunch of peaks’, should be Cruachan Bheann (but for which, see above), Watson and Murray (ibid.) suggest that Cruachan Beann may be a compound formation; however, a (closed) compound would give *ˈCruachan-ˌBheann, with a preposed, fully stressed specific element leniting a weakly stressed generic element.)
Further, note SG àrc in the sense ‘hollow near a cow’s tail by which you can judge when the cow is likely to calve’ 
An indication of imminent parturition in cows is the slackening of the sacro-iliac ligaments around the pelvis (NADIS), which creates a visible hollowing around the head of the cow’s tail.
Also called SG sloc an airich (ibid.), leg. sloc an àirich (cf. McDonald 1972, s.v.: ‘ “the grazier’s hollow”, a little hollow near the tail of a cow which indicates when cows are likely to calf’, South Uist). AFB˄ translates sloc an àirich (q.v.) erroneously as ‘vulva (of a cow)’.
Faclan bhon t-Sluagh˄, s.v. arcan: a’ bho a’ tighinn faisg air am breith – an arcan a’ fas farsuinn [sic] [‘the cow getting close to parturition – the arcan widens’], Skye, with an editorial note: ‘arcag?’, without further explanation.