v1.0
Publishing history:
v1.0: 01/10/24
pronnasg m. [ˈpʰɾɔ̃ᵰ̪əs̪k] ‘sulphur, brimstone’. Stewart (2004, 412) derives this word from ON brennisteinn (= brennusteinn, with the genitive of brenna f. ‘burning’ + steinn m. ‘stone’); McDonald (2009, 345) considers the loan uncertain. Although ON brennustein acc. would formally yield SG *breannaistean, which could account for some of the reflexes below, it is perhaps more likely that the word is a loan-blend from MScots brinstane, brynstan(e) etc., itself from northern MEng. idem (DOST˄), or directly from Middle English.
This may be the implication of MacBain’s (1911, s.v. pronnasg) ‘formed on Sc[ots] brunstane, [Old] Norse brennisteinn, Eng. brimstone’.
1. SG pronn- etc.
The first syllable is based on forms such as SG pronn vb ‘to pound, grind etc.’, 
Cf. pronnag ‘crumbs; rubbish’, pronnas ‘leavings of food, fragments’ (Dwelly 1911); pronnasg ‘crumbs, fragments’ (Wentworth 2003). SG pronn goes back to EG bronnaiḋ vb ‘spends, consumes; injures, damages’ (MacBain, s.v. †bronn; eDIL˄).
In addition to other forms cited above, Dwelly gives proinnistean and pruinneastan.
Dwelly also cites prannasg, with a ~ o alternation.
MacLennan (1925, s.v. pronnasg) cites Ir. pronnusc, so O’Reilly (1864), but this is mostly likely adapted from Kirk, so Dinneen (1947: pronnasc); contrast Ir. bromastún (Ó Dónaill 1977).
Pronnasg in the sense ‘sulphur’ falls together with pronnasg in the sense ‘fragments, crumbs etc.’ (e.g. Faclan bhon t-Sluagh˄, s.v. pronnasg, prannasg), the final syllable of which may be a development from the nominally abstract suffix -as (Cox 2017, 121; Thurneysen 1975, 168) + epenthetic g (see fn 2, above), cf. the sporadic development of -s > -sg elsewhere, e.g. SG taibhleas (taibhleis, taibhliosg, taimhliosg, tàimhleasg) beside tàileasg ‘chess; backgammon’ (Dwelly), EG táiḃlis etc. < OFr. tables (eDIL˄, cf. ON tafl nt. ‘board-game’); and tamhasg ‘uncouth fellow’ (Faclan bhon t-Sluagh˄: South Uist), ‘fool’ (Harris and Assynt), a variant of amhas, q.v. (for tamhasg in the sense ‘ghost’ (Barra and Harris), cf. tanas, tannas (> tanasg, tannasg) ‘idem’). The ending -asg in these examples coincides with the native -sk- suffix found for example in duileasg ‘dulse’, EG duilesc (Pedersen 1913 II, 19; Calder 1972, 181), and a number of words of unclear but possibly Scots provenance appear to be formed using this suffix or to be original plural forms that ultimately conform to it: brolasg ‘talkativeness’, cf. MScots brawl, braul vb ‘to boast loudly’ (DOST˄, s.v. 1brawl; cf. MacBain, s.v. brolaich); bùileasg ‘pothook’ (McDonald 1972, 53; Faclan bhon t-Sluagh˄) and bùlasg ‘idem; hooked pot handle’ (Faclan bhon t-Sluagh˄), cf. bùlas, bùlais ‘pothook’ (HSS), cf. Scots bools pl. ‘two crooked intruments of iron, linked together, used for lifting a pot by the ears’ (ibid.; SND˄, 2bool); crùilleasg ‘an old woman or instrument about to fall apart’ (Faclan bhon t-Sluagh˄: South Uist), cf. Scots crile, cryle, croil, croyle, cruyle ‘dwarf, dumpy deformed person; frail person, animal or thing broken down from age or ill-usage’ (SND˄); piolasg ‘a mischievous young boy’ (ibid.: Harris), cf. Scots pill ‘penis; (metaphorically) a young ling’ 
Cf. SG cnap, s.v., > cnapan ‘small lump; block; knob (penis)’ and cnapach ‘boy’.
SG truaill and Ir. truailligh ‘to corrupt’ are unconnected (see Vendryes 1996, s.v. trúailniḋ).
2. SG grunn- etc.
An alternative development, substituting the first syllable of the word with SG grunn- in the sense ‘grounds, lees, dregs’, 
Cf. grunnasg ‘dregs’ (HSS 1828; Watson 2022, 219: Easter Ross); grunndas ‘idem’ (Dieckhoff 1932: Glengarry), var. grullasg, grulasg ‘crumbs, fragments, broken bits’ (AFB˄; Faclan bhon t-Sluagh˄). For SG grunn ‘handful, lot, crowd’, cf. EG grinne ‘faggot, bundle; crowd, company, troop of soldiers’, Bret. gronn ‘heap[, pack, cluster, band]’ (MacBain 1911). Grunnasg in the sense ‘groundsel’ is derived by Clyne (1989, 18–19) from SG grunnd ‘ground’ but is probably a loan-shift from Eng. groundsel or Scots grundiswallow, gruniswall ‘groundsel’, conforming with grunnasg ‘dregs’.
For ‘sulphur’, Faclan bhon t-Sluagh˄ also records riof in Strathglass and riofa in Skye; cf. riof (Munro 1843, 6: riofa, 200: riof, gen. riofa, so MacBain 1911: riofa), riubh (HSS 1828 
Attributed to O’Reilly 1864 (1817), who also gives ruibh.