The term Syunik ( old Siwnik' ) is not attested in Urartian texts.

 The term Syunik ( old Siwnik' ) is not attested in Urartian texts. Instead of that we have the name Ṣuluqu and possibly Ṣiluquni. This name is attested later in Armenian period as Cəluk (Ծղուկ) as a canton of Syunik. But occasionally the term Cluk was also used as a synonym of Syunik. 

Jahukyan has proposed a Scythian etymology for the term. Tsuluk means upright, vertical in Shughni an East Iranian language.

Syunik region is not usually considered to be part of Etiuni ( and consequently part of Lchashen culture ) . We have an Iron Age sample from Kapan which is not different from Lchashen samples.

Despite good erymology it is very unlikely that it was a predominantly a Scythian place. Assuming that the term is Scythian this could simply mean that they became a ruling elite there. Later it seems a power shift occured and a new name was used the Siwnik'.

Various theories has been proposed for Siwnik'. 

Petrosyan linked it with Urartian sun God Shiwini.

Others proposed that it come from a term Abar-Siuni which is attested in Tighlat Palassar's campaign in Nairi. Albeit Abar - Siuni location is unknown.

Khorenatsi linked it with the name of Sisakan. I think the transition from Sisak to Siwni is unlikely from linguistic point of view. Despite this Syuni noble family was also called Sisakyan.

https://en.m.wiktionary.org/wiki/%D4%BE%D5%B2%D5%B8%D6%82%D5%AF#Old_Armenian