ONlwSG

ucas m. [ˈuʰkəs̪], [ˈu̟ʰkəs̪], gen. ucais [ˈuʰkiʃ] etc., ‘mature coalfish, Pollachius virens’ 

Armstrong 1825 has ‘a name given to an ass in its fourth year’: ass here may be short for jackass, which may have inadvertently been used for blackjack, a Scots term for ‘coalfish’.

has a number of forms:

A SG ugsa, usga, ugs, ugs’
A 1. SG ugsa [ˈuɡ̊s̪ə]
In HSS 1828; M^cAlpine 1832; MacEachen 1842; Forbes 1905, 46, 354; Faclan bhon t-Sluagh^˄: Harris, Taransay; AFB^˄: /ugsə/.

A 2. SG usga [ˈus̪kə]
In Forbes 1905, 46, 354: usca.

A 3. SG ugs, ugs’
(i) ugs [uɡ̊s̪] (Forbes 1905, 46, 354; Faclan bhon t-Sluagh^˄, s.v. ucas: [u̜ɡs], Sutherland);
(ii) ugs’ [uɡ̊ʃ] (Faclan bhon t-Sluagh^˄, s.v. ucsa, ucs(a): [u̟kʃ], [u̜kʃ], Sutherland).

B SG ucsa, ucas, umhcsa, omhcs, uiceas
B 1. SG ucsa [ˈuʰks̪ə]
In Armstrong 1825; McDonald 1972; Garvie 1999, 84; Faclan bhon t-Sluagh^˄: Lewis, Skye, Islay, Tiree; AFB^˄: /uxgsə/.

B 2. SG ucas [uʰkəs̪]
In HSS 1828, see ugsa; Forbes 1905, 46, 354; Dwelly 1911; Oftedal 1956, 360: /ukəs/; Garvie 1999, 84; Wentworth 2003: [uhkəs], Gairloch; LASID IV, 281: [u.kəs], Sutherland; Faclan bhon t-Sluagh^˄: Lewis, [u̜xkəs], North Argyll, [u̟kəs], Assynt; AFB^˄: /uxgəs/.

B 3. SG (nasalised) ucas, umhcsa, omhcs
(i) ucas, with a nasalised stressed vowel (Robertson 1904, 332, Wester Ross; also 1906c, 325);
(ii) umhcsa [ˈũks̪ə] (Faclan bhon t-Sluagh^˄, s.v. ucsa: [ũksə], Mull);
(iii) omhcs [õks̪] (Dwelly 1911: oncs, Arran).

B 4. SG uiceas [ˈuʰkʲəs̪]
In LASID IV, 258: [‘üχ´ḳ´əs], Lewis.

For SG ugsa and ucas, Henderson (1910, 121) compares ON uggi m. ‘fin’ and uggiðr, uggaðr ‘finned’, although these are only attested in modern Icelandic (Cleasby 1874); cf. Norw. dial. ugge ‘fin’ (Haugen 1984). Christiansen (1938, 4, 14–15) tentatively compares ON ufsi, noting the word is still found in West Norwegian dialects in the sense ‘mature coalfish’ (see Torp 1992, s.v. ufs). McDonald (2009, 424) considers a loan likely, without being more specific.

Lockwood (1962) notes alternation between ON fs ~ ps before the 11th century, 

Seip (1955, 293) notes differentiation (e.g. uxadropa) between 1300–1370.

hence ON ufsi ~ upsi m. ‘coalfish’, 

Cf. Ice. ufsi and Far. upsi ‘coalfish’; also ON refsa ~ repsa ‘to punish’ (Iversen 1973, 35).

and the sporadic development of p ~ k in some words, e.g. ON ups ~ ux (uks) f. ‘eaves’, so Norw. ufs, ups ‘eaves; precipice’, oks ‘precipice’ and Far. ups, uks ‘upper edge of a cliff’. Besides ON ufsi and upsi, then, Lockwood argues that an otherwise unattested ON *uksi, yielding *uks, would account for SG ucas and ucsa.

A late ON *uksa acc., then, might formerly yield both SG (A 1) ugsa and (B 1) ucsa, according to dialect (cf. Eng. hogshead > SG togsaid ~ tocsaid 

E.g. Faclan bhon t-Sluagh^˄, s.vv.

), which might in turn yield (A 2) usga and (B 2) ucas, with metathesis (cf. SG tosgaid ~ tocasaid 

E.g. AFB^˄, s.vv.

); apocope yields (A 3) ugs and, with a palatalised final, ugs’ (cf. Eng. box < SG bogsa ~ bogs ~ bogs’ 

E.g. Holmer 1938, 129, Ó Murchú 1989, 295, and Watson 2022, 125, respectively.

); nasalisation of the stressed vowel yields (B 3) (nasalised) ucas, umhcsa and, with o ~ u alternation and apocope, omhcs; 

For spontaneous nasalisation within a voiceless context in stressed syllables in Scottish Gaelic, see Ó Maolalaigh 2003, 109–17: 112.

and, finally, medial palatalisation yields (B 4) uiceas (cf. Scots breeks > SG briogais (s.v.) ~ brigis).