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sitir f. [ˈʃ iʰt̪ʲiɾʲ], gen. sitire -[ə], ‘neigh’ is derived by MacLennan (1925) from ON þytr m. ‘a whistling sound as of wind’ (cf. NO: ‘a whining, honking, humming, buzzing etc.’), so also Stewart (2004, 414); McDonald (2009, 430) considers the loan uncertain. ON þytr (acc. þyt) seems formally unlikely to yield SG sitir, despite Scots thoosand probably yielding SG sùs(t)an, q.v.
SG sitir goes back to EG sitrech, seitrech (also sitir) ‘a neighing; act of neighing, braying; a sneezing’ (eDIL˄, s.v. sitreach). EG sitir is probably onomatopoeic in origin (Vendryes 1996, s.v. sitrech), which, with suffix and syncope, yields sitrech, seitrech. The feminine suffix -ach/-ech is frequent in expressive verbal nouns, e.g. Ir. búireach ‘bellowing’, sraothartach ‘sneezing’, seitreach ‘neighing; neigh, whinny; snort’ (Ó Dónaill 1977). 
Also seitríl and seit(i)reacht, with alternative suffixes (ibid., s.v.).
For final -i, cf. O’Rahilly (1976, 57), who notes that ‘[in] the extreme south of the Scottish Gaelic area (e.g. Arran, Kintyre, Rathlin) -ich is usually pronounced -i’, cf. SGDS Items 71–72 balaich, Points (Arran) 31–35, (Kintyre) 37, 39, 41–42, and (Islay) 54–56.
For final /i/, see fn 2, above.
SG sitir itself is listed in HSS 1828; MacEachen 1842; Dwelly 1911; MacLennan 1925; Dieckhoff 1932: [Sicdˈjirˈ], Glengarry; AFB˄: /ʃ ihdʲɪrʲ/; and Wentworth 2003, s.v. neigh: sitir, but no pronunciation given, Gairloch; while Faclan bhon t-Sluagh˄ lists sitir for North Uist, but sitear [ʃ itʹəɾ] for Tiree and siotar for South Uist, cf. McDonald 1972, s.v. sidir (editor: sitir): also siotar, and Mac Gill-Fhinnein 2009, 130: siotar [ʃ ihtər], both South Uist.
SG sitrich is given the senses ‘neighing, braying; sneezing’, but also ‘tittering’ (Dwelly 1911), ‘sniggering; whispering’ (Wentworth 2003, s.vv.: Gairloch) and ‘snorting’ (Faclan bhon t-Sluagh%%, s.v. seitrich: Harris), while SG sitir is given the senses ‘neigh, bray; neighing, braying’, but also ‘an obstreperous laugh’ (HSS 1828).
Derivatives: SG sitrich vb (Dwelly 1911; AFB˄), and the agent noun sitriche (Dwelly 1911). The adjective sitreach is listed in HSS 1828: sit(i)reach; McAlpine 1832; MacEachen 1842; Dwelly 1911: sit(i)reach; and MacLennan 1925, but is apparently otherwise unattested: it may be the result of a rationalisation of sitreach/seitreach as found in Kirk 1702, Shaw 1780 and Mac Farlan 1795, but may really be a ghost word in Scottish Gaelic.
Instances of [i] ~ [e] alternation in the stressed vowel of some of the above forms may be due to confusion with EG séitiḋ ‘blows etc.’, Ir. séid, SG sèid 
With which MacBain (1911) compares SG sitir, sitrich and Ir. sitreach.
Cf. séidrich ‘blowing; panting; anhelation; blustering, as of wind’ (Armstrong 1825); seidrich [shājjˊ-rėch] (i.e. with [eː]) ‘hissing of serpents’ (McAlpine 1832); séidrich ‘hissing of serpents’ (Dwelly 1911, after McAlpine); sèidrich ‘hissing of serpents etc.’ (MacLennan 1925); séidrich /ʃ eːdʲrʲɪç/ ‘hissing’ (AFB˄).
Cf. sèitrich ‘blowing, puffing’ (MacEachen 1842); séitrich (Dwelly 1911: see séidrich).
Where the penultimate letter ?c has been overprinted with ?d, or vice versa.