Publishing history:v1.0
v1.0: 05/06/26
slabhairc f. [ˈs̪ɫ̪ɑvaɾ̥ʲkʲ], [ˈs̪ɫ̪ɑvəɾ̥ʲkʲ] 
Cf. (Harris) [sɫɑvɑðc], (North Uist) [sɫɑu̜əðc] (Faclan bhon t-Sluagh˄), /sLa.ɪrʲçgʲ/ [sic] (AFB˄).
Mackenzie’s slo is unattested in Old Norse and is from Cleasby’s (1874) Ice. sló f. ‘the bone in the hollow of the horns of animals’. SG slabhairc is otherwise slabhag ‘the pith of a horn; the socket of a horn; a deer’s first antler’ (cf. Armstrong 1825; HSS 1828; McAlpine 1832; Dwelly 1911: also sladhag ‘keratin, outer sheath of the horn of an ox, sheep etc.’, slodhag ‘the lining of a horn’; MacLennan 1925; Dieckhoff 1932; Faclan bhon t-Sluagh˄: Scalpay, Skye: [sɫɑu̜əɡ] and slathag; 
Presumably simply an orthographic variant of sladhag, and not connected with SG slathag in the senses ‘handful of corn cut at one stroke of the reaping-hook’ and ‘snowflake’, for which s.v. slag fn 4.
But note that LASID IV, 211, records slàbhag [sɫɑːvɑk.] for Arran. AFB˄ also lists the diminutive slabhagan /sLa.agan/ in the sense ‘pith of a horn’. (For slabhagan is the sense ‘type of algae’, s.v. slòc).
MacBain (1911) suggests that slabhag might be from Scots sluch ‘the outer skin or husk of a fruit or vegetable etc.’ (SND˄, s.v. sloch), but it is probable that slabhag is a variant of SG *slabhac (with the same meaning), with the final syllable altered on the analogy of words ending in the (diminutive) suffix -ag; for *slabhac, otherwise slòc (q.v.), cf. Ir. sleabhac, (Donegal) slabhac ‘horn, especially at the initial stage etc.’ (Dinneen 1927), and EG sliḃac ‘the pith of a horn’ (eDIL˄, s.v. slibac), probably the same word as *sleḃac (cf. slebcán ‘?an edible seaweed’ (eDIL˄)), perhaps from the root (s)leib- ‘slimy, slippery etc.’ (Pokorny 1959 II, 663). Meanwhile, slabhag (*slabhac) has most likely been transformed into slabhairc, where it occurs, under the influence of the final of SG adhairc. 
Not by the process that Mackenzie (≈ibid.) suggests: ‘[t]he term seems to be a kind of hybrid. Slo in Norse has the same meaning; adhairc must have been added when the meaning of slo began to be forgotten.’