v1.1
Publishing history:
v1.0: 01/10/24
v1.1: 06/08/25
sainnseal m. [ˈs̪ãı̃ʃɑɫ̪], gen. sainnseil -[al], 
Cf. (Gairloch) [s[ɑ̃ı̃]ʃɑʟ] (Wentworth 2003, s.v. lesson), (Glengarry) [s(ai)*Sal] (Dieckhoff 1932), cf. [seyshal] (MacLennan 1925), but /sãĩNʲʃəL/ (AFB˄).
Craigie (1894, 159) suggests SG sainseal (sic) is a loan from ON handsal nt. ‘handshake, esp. to confirm an agreement; a promise or security given with a handshake’ (NO), 
Without explanation, McDonald (2009, 360–61: sainseal) defines ON handsal as ‘a new-year gift, with the sense of reciprocal obligation’, but considers the loan uncertain.
HSS (1828) takes SG sainnseal to be a derivative of SG seamhas [ˈʃɛ̃vəs̪] (cf. McAlpine 1832: [sév´-us], although MacLennan (1925) gives [shēvus] with a long stressed vowel, possibly via confusion with sèamh adj. ‘mild, peaceful’) ‘good luck’, but seamhas is an Argyllshire variant of seanns, from Eng. chance (MacBain 1911; MacLennan 1925), via back-formation, cf. SG dannsadh ‘dancing’ [ˈd̪̥ãũs̪əɣ], (Islay) /dɛvəsəɣ/ (based on Eng. dance), cf. SG samhradh ‘summer’ [ˈs̪ãũɾəɣ], (Islay) /sɛvərəɣ/ (EG saṁraḋ) (Grannd 2000, 56–57; Cox 2008a, 54–55).
SG sainnseal in the sense ‘gift’ is extended via ‘reward’ to ‘a lesson learnt, consequence’ (Wentworth 2003, s.vv. learn, lesson, story: Gairloch) and to ‘blow, shock’ (Faclan bhon t-Sluagh˄, s.v. sainnseal: ‘verbal drubbing’, Harris; ‘blow, slash’, Harris, Scalpay; ‘unexpected shock’, North Uist; ‘severe blow’, Skye).
Henderson (1898, 209, stanza II.1, and 211) notes that SG mar shanns’ ‘as a gift’ 
In a poem by Fear na Pàirce (MacCulloch of Park, Strathpeffer), fl. late-16th to early-17th century.
More recently borrowed as seans(a) and teans(a). See also fn 3, above.
Derivatives: SG sainnsealaich m. (e.g. HSS 1828) and sainnsealaiche m. (e.g. Armstrong 1825) ‘one who gives a handsel’, and sainnsealach adj. (e.g. HSS).