v1.0
Published 01/10/24
briogais f. [ˈb̥ɾʲiɡ̊iʃ], [ˈpʰɾʲiɡ̊iʃ], gen. -e [ə], ‘trousers, breeches’ is derived by Stewart (2004, 408) 
Misreading MacLennan (≈1925), who gives ‘m. “breeches”. Ir. brigis. Scots breeches. OEng. brēc (pl.). ON brækr (pl.)’.
from ON brækr f., although McDonald (2009, 346) considers the loan unlikely because of the disparate vocalisms involved. ON brók sing. usually refers to ‘one leg of a pair of trousers’, while brǿkr pl. (or brœkr, not brækr) usually refers to ‘a pair of trousers’; brók would formally yield SG *[b̥ɾɔːɡ̊], while brǿkr would formally yield SG *[ˈb̥ɾeːɡ̊əɾ], which would most likely develop into *[ˈb̥ɾiaɡ̊əɾ] in more westerly and northerly dialects (Jackson 1968, 65–71; Cox 2017, 7 fn 20). SG briogais is most likely from Scots breeks [briks] ‘trousers’ (SND˄, s.v. breek) or Northern Eng. breeks, as opposed to Eng. breeches.
MacBain (1911) derives SG briogais from Eng. breeks, breeches; as does Oftedal (1956, 82). Modern Eng. breek is from OEng. (pl.) brēc, and breeks is therefore a double plural.
MacLennan’s Ir. brigis appears to follow MacBain in citing O’Reilly’s Irish-English dictionary’s ‘brigis f. “breeks, breeches”’, 
Listed as part of the main dictionary in the 1864 edition, but within the Supplement of the first, 1817 edition.
but this is probably from Shaw’s Scottish Gaelic-English dictionary (1780: brigis ‘breeches’). SG brigis, with a palatalised medial plosive, is an Argyllshire form of briogais: bríshnin [see below] no brigis ‘breeches’ (≈MacDomhnuill 1741, 18); ‘Oran do‘n Bhrigis’, the original form of the title of Donnchadh Bàn Mac an t-Saoir’s song referencing the Disclothing Act of 1747 (MacLeod 1978, 8, 427); brigis, Kintyre (Holmer 1957, 144); brigeis [bɾıɡʹəs] (sic), Islay (Faclan bhon t-Sluagh˄); ?and brekish (sic), ?Arran (ibid.).
Contrast Ir. bríste f. ‘breeches, trousers’ (EG briste and, without metathesis, braitsi), from Eng. breech. Note also SG (pl.) bríshnin (MacDomhnuill 1741, 18) and (dat. pl.) brìsnibh (Mac-Dhonuill 1751, 124, line 20 
From ‘Oran nam Bodach’, rhyming with ínsgin : tíres’ : grís-fhiunn.
) ‘breeches’. Although described as the plural of briogais (HSS 1828: brisnean; Dwelly 1911: idem), brìsnean is conceivably a gaelicisation of Eng. breeches, with sn for st (cf. SG slèisne for slèiste, gen. of sliasaid f. ‘thigh’) and substitution of the Gaelic plural ending -an.