Publishing history:v1.0
v1.0: 16/01/25
sìoman 
Occasionally spelt sioman, siaman, simean (e.g. Faclan bhon t-Sluagh˄, s.vv.)
m. [ˈʃĩə̃man], 
E.g. (Lewis) [ʃ[iə]mɑn] (Borgstrøm 1940, 43), /ʃĩə̃mɑn/ (Oftedal 1956, 96, 120–21), [ʃiəman] (Oftedal 1972, 120) and (in the dative after the article) [ən´ ˈhi.əman] (LASID IV, p. 256, Item 991); (East Perthshire) /ši·əmɑn/ (Ó Murchú 1989, 401; cf. Robertson 1900, 12–13: with ‘i and close a’, Perthshire); (Gairloch) [ʃ[ĩɑ̃]mɑn] (Wentworth 2003, s.v. rope); (Glengarry) [Siaman] (Dieckhoff 1932, s.v. sioman); (Islay) [shēm´-an’] (McAlpine 1832); and, in weakly stressed position, (Lewis) [ʃiːəman kɔũʟɪx´] ‘of straw’ (LASID IV, p. 259, s.v. connlach) and (Mull) [ʃimɑ.nɸrəəç], i.e. [ˌ ʃimɑ.n ˈɸrəəç] ‘of heather’ (Faclan bhon t-Sluagh, s.v. simean fraoich). However, the stressed vowel is short even in stressed position in (Easter Ross) /ʃimɑn/ (Watson 2021, 18; 2022, 290, s.v. sioman), see below. In the case of (Sutherland) [ʃi.əṽənh fröːx] ‘of heather’ (LASID IV, p. 276, Item 528), the form is taken to be for [ʃi.ə̃mənh fröːx], perhaps on the analogy of SG siobhag ‘straw; wick’, s.v.
gen. sìomain -[æɲ], -[ɛɲ], ‘rope made from plant fibres (esp. straw but also coir, heather, grass, twigs or rushes)’ (AFB˄), also ‘a tall, shapeless fellow’ (Dwelly 1911), is generally derived from ON sími m.
Mackay 1897, 95: sioman < Ice. simi [sic]; Christiansen 1938, 5, 20: sioman; Borgstrøm 1940, 43; de Vries 1962; McDonald 2009, 298.
or síma nt. ‘rope, cord’,
Craigie 1894, 158; Henderson 1910, 117 (so also Ó Baoill 1978, 96 §8.9); MacBain 1911; MacLennan 1925 (so also Stewart 2004, 414); McDonald 2009, 298: ON sima [sic].
the oblique form of both being síma, the case form proposed by Oftedal (1956, 96, 120–21; 1972, 120; 1983, 155). ON síma appears to have yielded SG sìoman via the addition of the nominally diminutive suffix -an, although Henderson (1910, 117), MacBain (1911) implicitly and McDonald (2009, 298) explicitly derive the final syllable in Gaelic from the Old Norse genitive plural ending (símna), which would formally yield SG -[ən] via metathesis, or -[an] by analogy with the diminutive suffix.
O’Reilly’s (1817) Irish dictionary lists sioman [sic] ‘a rope, a cord’, but it is possible the word was adopted from Shaw’s (1780) Scottish Gaelic dictionary: sioman ‘rope, cord’. Dinneen (1947) lists Ir. síomán in the sense ‘rope or cord’ (after O’Reilly), but also ‘a strap used to bind a sheaf (Ulster)’, cf. ‘wisp of straw (as binding for a sheaf)’ (Ó Dónaill 1977).
Ó Baoill (≈1978, 96 §8.9) notes that the ‘word is listed in O’Rahilly 1976, 185, as a Scottish word found in [South] East Ulster’, but that he failed to find O’Rahilly’s evidence for the existence of the word there. O’Rahilly may have based his comment on Dinneen’s entry.
Ir. síomán seems likely to be a borrowing of SG sìoman.
Marstrander (1932) considers Mx oalsym, ousym ‘a fetter tying the head of an animal to its forelegs’ 
Cf. Cregeen 1835: oalsum ‘a tie on a thievish cow, a rope tied from the horn or head to the leg’; Y Kelly 1866: ousym ‘a shackle, a bond, especially that cord which binds a wild bull’s horns to his forelegs’.
to derive from either (p. 49) ON *hǫfuð-sími ‘head-rope’ or (p. 265) *hald-sími ‘hold-rope’, but the Manx forms conceivably represent ON *halssími ‘neck-rope’, perhaps via northern Eng. *halse-sime or *hause-sime ‘idem’, cf. hause-band ‘collar, necklace’ (EDDo˄, s.v. halse), Far. háls-band ‘necklace, (dog) collar’.
Scots shimee [ˈʃɪmi] ‘a straw rope’ is taken to be a scotticised form of SG sìoman, while Scots sheemach, sheemich [ʃiməx], dim. sheemachan, ‘a piece of thick matted cloth or dress, a tangled mass of hair or weeds’ is also (though more tentatively) connected with sìoman (SND˄, s.vv.).
Watson (2021, 18) is more confident: Scots sheemach with a ‘suffix identifying this form as a Scots borrowing from Gaelic’, although -ach here is potentially a variant form of the diminutive Scots ending -ock (SND˄, s.v. -och II.2.(4)).
Scots simmen [′sɪmən], (Shetland also) [′sɪmə(n)t] ‘a rope made of heather, grass, rushes or especially straw’, on the other hand, is thought to go back to ON síma, although SND˄ notes that its exact formation is uncertain.
ON sími (or síma) might formally be expected to yield Scots *sime *[səim], cf. northern Eng. sime [saim] ‘idem’ (EDDo˄). This would explain the non-palatal initial [s]- of Scots simmen. However, a long stressed vowel in Old Norse would normally be expected to yield a long (*[əi]) rather than a short stressed vowel ([ɪ]) in Scots, and to explain the short stressed vowel in simmen Jakobsen (1928) compares Swedish dialectal forms with short i and long m (e.g. simme ‘rope, twine, tie (in southern Sweden, made of cattle, goat or pig hair)’ (Rietz 1867, s.v.)). The final syllable of Scots simmen possibly represents a derivation in -in(g), but ‘there may have been some influence from SG sìoman and Ir síomán ... The -nd forms arise from -n(n) by reverse analogy ..., and become -nt and finally -t, hence simmet. Jakobsen’s explanation that -en represents the suffixed article of sími m. and -et of síma nt. is hardly feasible’ (≈SND˄).
Potential influence from SG sìoman on Scots simmen is understandably considered to be limited to the final syllable (e.g. SG clachan [ˈkʰɫ̪ɑxan] ‘village’ > Scots clachan [ˈklɑxən] ‘idem’), principally because the Scots word has a non-palatal initial and a short stressed vowel, while the Scottish Gaelic word has a palatal initial and a long stressed vowel. However, as EG crínaiḋ ‘withers’ (> SG crìon [kʰɾʲĩːn], [kʰɾʲĩə̃n] ‘to wither’) yields MScots cryne, Scots crine [krəin], also (Caithness, Banffshire, Angus) [krin], ‘to shrink’, perhaps SG sìoman could have resulted in a short stressed vowel in Scots.
The short stressed vowel recorded for Easter Ross Gaelic (Watson 2022, 290, s.v. sioman: /ʃimɑn/) is likely to be the result of Scots influence, or perhaps conflation with SG seaman ‘tail’, which has a short stressed vowel; contrast (North-West Sutherland) sìoman (Grannd 2013, s.v. rope), with a long vowel.
Speculatively, then, a loan-blend of early modern SG [ˈʃĩːman] into Scots might conceivably yield [ˈsɪmən], with an original Old Norse loan in Scots (*sime) partially adapted to the Gaelic word.
Derivatives: 
From, for example, Faclan bhon t-Sluagh˄ and AFB˄.
SG sìomanach adj., with the suffix -ach; sìomanaiche m., with the agentive suffix -aiche; the verb sìomanaich; and the verbal nouns sìomanaich (also siamalaich, with dissimilation) and sìomanachadh.