ONlwSG

udabac m. [ˈud̪̥ə ̩ b̥aʰk] and ùdabac [ˈuːd̪̥ə ̩ b̥aʰk], 

Cf. [ûd´-a-bŏchg] (M^cAlpine 1832). In AFB^˄, however, the ending has been assimilated to the Gaelic diminutive suffix -ag -[aɡ̊]: ùdabac /uːdəbag/.

gen. idem, 

Although AFB^˄ gives ùdabaic.

‘outhouse etc.’ is derived by MacBain (1896) from ‘Norse’ úti-bak, tentatively translated as ‘out-back’, but which is attested in neither Old Norse or Icelandic; MacBain is followed by Henderson (1910, 114) and McDonald (2009, 425). ON *uti-bak would formally yield SG *uideabag *[ɯd̥ʲəb̥əɡ̊], or *[ɯd̥ʲəb̥aɡ̊] assuming assimilation with the Gaelic diminutive suffix -ag.

SG udabac, with a short stressed vowel, is given in HSS 1828, and by MacBain, Dwelly (1911) and Henderson. MacBain, Dwelly and Henderson also cite ùdabac, with a long stressed vowel, for Uist. M^cAlpine (1832, s.v. udabac: [ûd´-a-bŏchg], Islay), MacLennan (1925), Faclan bhon t-Sluagh^˄ (s.v. ùdabachd: Skye) and AFB^˄ also cite the long-vowel form.

M^cAlpine simply gives the sense ‘porch’, HSS ‘backhouse, porch’, MacBain ‘outhouse, porch, backhouse’, Henderson ‘outhouse, backhouse’ and MacLennan ‘porch, outhouse, backhouse’. Dwelly is far more detailed and gives ‘porch, outhouse; a buttress or support, usually of stone, built against a wall; a wall about 6 ft high and 7 ft or 8 ft long, built in front of a main door at a distance of about 4 ft from it to break the force of the wind (they were common in the north of Skye till quite recently and may still be seen in some parts) [cf. Faclan bhon t-Sluagh^˄: ‘a roofless semicircular wall made of drystone which was constructed as a primitive porch or windbreaker to give shelter to the main outside door of a dwelling’, Skye]; backhouse’.

It seems probable that SG udabac and ùdabac are reflexes of Scots *outback *[utbɑk], a compound of out- + back (for bauk), perhaps originally with reference to the partition between the inby and outby, i.e. the inner (fireside) and outer areas within a building (cf. Marwick 1929, s.v. back, sense 7). For variation in the length of the stressed vowel, cf. SG udrathad ~ ùdrathad (s.v.).