Publishing history:v1.0
v1.0: 05/06/26
slabhcar m. [ˈs̪ɫ̪ɑuʰkəɾ] in the sense ‘spiritless fellow’ is derived by Mackay (1897, 91: slaucar) from Ice. slakr ‘idle’ (cf. Cleasby’s (1874) Ice. slakr ‘slack’), but by MacBain (1896: slaucar ‘slouching fellow (Sutherland), taunter’; 1911: slabhcar ‘idem’), Henderson (1910, 212–13: slabhcar ‘slouching fellow, one given to taunting’, 346: slabhcair, 351: slabhcar) and MacLennan (1925: slabhcar ‘slouching fellow, taunter’ 
So also Stewart (2004, 414), but who translates ON slókr as ‘insult’ in error.
Cf. ON slókr m. ‘a dull, heavy, lazy person’ (NO).
Note that Gunn’s (1900b, 120) rewording of MacBain (1911) implies something somewhat different: ‘slabhcar, awkward fellow, from Norse, slokr, and English, slouch’ [sic]. OED˄ notes that the origin of Eng. slouch is obscure, comparing Eng. slotch and slouk in the same sense, the latter compared with Norw. slōk and Ice. slókr. One of the two citations given for slouk, however, is from Wexford (1867: ‘an idle, heedless person’), and may in fact be a reflex of Ir. sleabhac, s.v. slòc.
Mackay’s form slaucar 
Mackay’s form appears in a paper published in 1897, first presented in February 1895.
Potentially also [ˈs̪ɫ̪ɔːʰkəɾ] (cf. SG slabhcan ~ slòcan, s.v. slòc).
Dwelly 1911, s.v. slaucar: see slabhcar; s.v. slabhcar: ‘slouching fellow; taunter; spiritless fellow (Sutherland)’. Duncanson (1897, 10) notes the question of the unusual orthography: ‘The Norse slókr “slouching fellow” is regarded by both Mr Gunn and Mr MacBain as the original of a word that is very strangely spelt for a Gaelic word, viz slaucar “an awkward fellow”. Where is there a precedent in the Gaelic language for the combination au?’ (A Strathglass contributor to Faclan bhon t-Sluagh˄ maintains (incorrectly) that slaucar ‘sloucher, dawdler’ is misspelt slabhcar, and that there was ‘never a “b” [sic] in this word’.
It occurs in an anecdote associated with a Mull-based account of the devil-summoning ceremony taghairm (Black ibid., 169).
Note that AFB˄ takes Campbell’s slauchdan as slabhcan (pers. comm. Michael Bauer, AFB’s editor), rather than slabhcar, but with a definition identical to that of SG leisgean, leisgeadair, leisgear, viz ‘lazy/idle person, sluggard, slouch(er), layabout, idler, lazybones’. (For slabhcan in the sense ‘type of algae’, s.v. slòc.)
Ice. (or ON) slak, the stem form of slakr, would formally yield SG *slag *[s̪ɫ̪ɑɡ̊], while ON slók acc. would formally yield SG *slòg *[s̪ɫ̪oːɡ̊] or [s̪ɫ̪ɔːɡ̊]; neither would be expected to yield SG slabhcar. Slabhcar (Mackay’s slaucar) may simply be for SG slabhcan, 
Duncanson’s (1897, 10) suggestions include a connection with the related SG slabhag ‘the pith of a horn’ (s.v. slabhairc) and slabhagan ‘a type of algae’ (s.v. slòc). He also suggests a connection with Scots slawk ‘a type of algae’, for which s.v. slòc.
The nature and origin of the suffix is problematic. The word slabhcar seems to be associated with Sutherland, where an original -air -[aɾʲ] could yield either -[aɾ] or -[əɾ] or both (cf. SGDS Item 393, Points 139–150); cf. *duilear (s.v. duil fhear) and ealbhar, q.v. On confusion between the agentive suffixes -air and -aire -[əɾʲə], see Ó Maolalaigh 2013, 210–13.