ONlwSG

brùc m., sometimes f., 

E.g. Badenoch (Dwelly App.: bruchd).

[b̥ɾu̟ːʰk], [b̥ɾuːʰk], gen. brùca -[ə], ‘seaweed cast ashore’. SG brùc has been derived from ON brúk nt. ‘dried heaps of seaweed, (NO) heap of seaweed on the shore’, 

Mackenzie (1910, 384); MacBain (1911); Christiansen (1938, 3, 9–10: bruc, leg. brùc); de Vries (1962); McDonald (2009, 346); and Ó Muirithe (2010, 22). MacLennan (1925) merely compares SG brùc with the Old Norse word.

as have bruchd [sic], 

McDonald (2009, 346).

bruic [sic], 

Mackay (1897, 94), who cites Ice. bruk [sic]; Henderson (1910, 143); and McDonald (2009, 346).

bruga and briùig. 

Henderson (1910, 143).

EG brúcht ‘a burst, bursting, breaking out; pouring out; belching’ (eDIL^˄) yields SG brùchd [b̥ɾu̟ːxk], [b̥ɾuːxk] regularly.

E.g. Easter Ross (Watson 2022, 129: /bruːxg/); Glengarry (Dieckhoff 1932: [bruːKg]; West Perthshire (Ó Murchú 2021, 172: /bruːx(g)/; Islay (M^cAlpine 1832: [brûchg] (= [bruːchg])).

However, in some dialects brùchd alternates with or is replaced by brùc, 

For example in Sutherland (Faclan bhon t-Sluagh^˄, s.v. brùchd: [bɾu:k]); Lewis (ibid.: [bɾu̟:k], ?leg. [bɾu̟:ʰk]); Gairloch (Wentworth 2003, s.v. belch: [b̥ruːʰk], [b̥ruːxk]).

a back-formation by analogy with words such as muc, whose pronunciation can be generalised as [mũk], [mũʰk] or [mũxk], according to dialect, depending on the level of preaspiration before the final consonant (SGDS Item 633; see also O’Rahilly 1976, 148–49 and Robertson 1908a, 274).

Contrast the opposite development in SG sloc (< ON *slǫkk nt. ‘hollow’) > slochd (Cox 2022, 775–76, fn 517).

SG brùchd and brùc are also found in the sense ‘heap, a large quantity of something’ and the sense ‘seaweed cast ashore’ is probably a further extension: brùchd in Lewis and Skye (Faclan bhon t-Sluagh^˄); brùc (ibid.; MacLennan 1925) in Lewis (Mackenzie 1910, 384; MacBain 1911; Christiansen 1938, 3, 9–10: bruc, leg. brùc). A normalised dative form brùic is also found: in Sutherland (Mackay 1897, 94: bruic; 

Mackay disregards lengthmarks.

so Henderson 1910, 143 and Dwelly App.) and Strathglass (Faclan bhon t-Sluagh^˄).

ON brúk nt. would formally yield SG *brùg *[b̥ɾu̟ːɡ̊], *[b̥ɾuːɡ̊]; 

A possible variant bruk (NO) would formally yield SG *brug, with a short vowel.

it does yield Scots brook, bruk [bru(ː)k] (SND^˄, s.v. ²brook), which may lie behind SG bruga 

Cf. bruga dubh (Dwelly 1911, who refers to bruchda dubh, although the latter is listed as brùchda dubh).

and a normalised dative form brùig attested in Eriskay (Faclan bhon t-Sluagh^˄) and South Uist (ibid.; McDonald 1972, 51 

Distinct from brùchd ‘belch’ (p. 204).

).

Accordingly, ON brúk and SG brùchd (along with its variant brùc and the normalised dative form brùic), while they have independent origins, are semantic parallels, but SG bruga (along with the normalised dative form brùig) derives from Scots brook, bruk, itself from ON brúk. It is possible, of course, that Old Norse usage was the catalyst for the extension of SG brùchd to ‘seaweed cast ashore’, and/or that ON brúk (or bruk) was indeed borrowed into Gaelic but adapted in some dialects by morphemic substitution to the native brùchd, but there seems to be no way of confirming either hypothesis.

Faclan bhon t-Sluagh^˄ also lists the forms bruchd and, for South Uist, bruchda-dubh (cf. fn 11, above) and bruig without lengthmarks; cf. Dwelly App.’s bruchd for Badenoch (fn 1, above). If these are not in error (and not influenced by Scots brook, bruck), the short vowel may be the result of confusion with SG ruc, ruchd ‘a rick of hay or corn; a stack of peats’, from Scots ruck (s.v. rùc), and/or SG luchd in the sense ‘load etc.’ (EG lucht). Forms with palatalised r are occasionally found: (Lewis) briùchd /bðuːxɡ/ (Oftedal 1956, 103); and (Tiree) briùig (Henderson ibid.).