ONlwSG

rubadh m. [ˈɍu̟b̥əɣ], 

Cf. Faclan bhon t-Sluagh^˄: [ru̜bəɣ], Lewis.

gen. rubaidh -[i], ‘a type of earmark, in the form of a vertical cut down the tip of the ear’.

In enumerating sheep earmarks traditionally used in Lewis, MacArtair (1953, 76) lists rubadh sa bhun (sgoltadh) [‘a rubadh in the top (a slit)’].

MacMillan (in Christiansen 1938, 5, 26) compares SG rubadh 

Chunnacas mult connadel le toll anns a chluais deis agus rubadh anns a chluais taisgeil [sic] (ibid., 5) [‘A stray sheep with a hole in its right ear and a rubadh in its left was seen’].

with ON rippa (?), but Christiansen suggests ON rifa (leg. rífa) ‘to rend, tear’. McDonald (2009, 395) is right to be doubtful of the phonetic development: 

McDonald compares the development ON raufa ‘to break up or open’ > EG ruḃaḋ ‘the act of killing’, as tentatively suggested by Stokes (1900, 92), but the development can be rejected (so Marstrander 1915a, 125): ON au would be expected to yield a long vowel in Early Gaelic (ibid., 71–72).

neither ON i or í would be expected to yield /u/ in Scottish Gaelic; Old Norse geminate -pp- would be expected to yield SG -[ʰp]-; and ON -f- would normally be expected to yield SG -[v]- or hiatus.

SG rubadh is most probably a loan-blend from Scots rup in the sense ‘a sheep-mark, being a narrow slit cut in the ear’ (a variant of rip (SND^˄, s.v. rip v., n.1 II.2)) + the common Scottish Gaelic verbal noun ending -adh. Instead of rubadh, Dwelly (1911) lists ròibeadh, 

Silently altered to roibeadh in AFB^˄.

probably in error, cf. AFB^˄: roibeadh /Rɤbəɣ/, and North Tolsta Historical Society^˄: roibeadh.