ONlwSG

²trosg m. [t̪ʰɾɔs̪k], 

Cf. /trɔsd/ (AFB^˄).

gen. ?idem, in the sense ‘thud or crack’ is derived by Mackay (1897, 92: Sutherland) from ‘Ice. prosk [leg. þrösk] “a noise, beating as from threshing” ’ (after Cleasby 1874), cf. ON þrǫsk nt. ‘noise, racket’, both words related to ON þreskja ‘to thresh’.

McDonald (2009, 393) considers the loan unlikely on the basis that Mackay’s Icelandic form is unattested in the range of standard lexical works and because the final cluster of Dwelly’s tost differs from Mackay’s tosg.

While ON þrǫsk would indeed be expected to yield SG trosg via back-formation, 

ON þ- [θ] in a non-palatal environment gives EG th- [θ] (later developing into [h]), the lenited form of EG t- [t̪]; cf. ON Þormund acc. > SG Tormod masc. personal name, and ON þorsk acc. > SG trosg ‘cod’, with metathesis, s.v. (Cox 2022, 207–08).

Mackay’s trosg is more probably a by-form of SG trost (previously also written trosd) ‘knock, fall, stroke, as of one striking the ground after falling from a height’ (Dwelly 1911: Wester Ross).

Cf. SG cost ~ cosg < Eng. cost, and SG balaist ‘ballast’ ~ ballaisg ‘boaster’, s.v. ballart.

Cf. MacLennan 1925, s.v. trosd: ‘dead weight, heavy load’; McDonald 1972, s.v. trosd: ‘thud, heavy blow’, South Uist; Wentworth 2003, s.vv. bang, bump, strike, thump, Wester Ross; Faclan bhon t-Sluagh^˄, s.v. trosd: ‘thump’, Scalpay; AFB^˄: ‘bang, thud; bump, strike’. Compare Mackay’s phrase ≈thuit e le trosg ‘he fell with a crash’ with bhuail Jones le trost air an talamh chruaidh (Mac-Talla VII, No. 38, p. 4) ‘Jones hit the hard ground with a thud’ and leis an eagal gun robh an duine a bha na throst air an rathad marbh (MacFhearghuis 2014, 235) ‘fearing that the man that was in a heap on the road was dead’.

For SG trost, cf. Ir. trost ‘noise, report; thud, thump; tramp (of feet)’ (Ó Dónaill 1977), 

Cf. Lhuyd 1707: trosta ‘a crack’; O’Brien 1768: idem; O’Reilly 1817: trost, trosta ‘crack, noise’.

both of which go back to EG trost ‘noise, report, cry’ (eDIL^˄), which O’Rahilly (1946, 36–67) derives from a P-Celtic word cognate with the Pictish personal name Drust.

≈Ibid.: ‘Drust I would equate with W trwst “noise”, Bret. trouz “bruit, tapage”, from *trustu-’; and fn 7: ‘Ir. trost “loud or thunderous noise, as of a falling body striking the ground” (cf. deilm .i. torand nó throst (Leḃor na hUiḋre, in Best and Bergin 1929, 19, line 542)) must be a borrowing from Ivernic [the pre-Goidelic language spoken in Ireland before Old Irish (O’Rahilly, ibid. 88–91: 90)]; the -o- suggests that it represents a Celtic by-form *trusto-.’ GPC^˄ describes the exact relationship between W trwst, trws, MC and C tros, MBret. trous, Bret. trouz and EG trost as uncertain.