See also dy adv. and dyn pron., and ˀykdyn adv.
From an etymological perspective, this ubiquitous Syriac enclitic is almost certainly the heir to earlier ˀdyn (rather than a similar but limited usage of the pronoun dyn in non-Syriac literary texts, which itself is rather a reflection of Biblical Hebrew usage), but in Syriac usage it has become a calque of the Greek enclitic δέ. Also see s.v. hydyn.
The whole issue is complex: See A.M. Butts, "Between Aramaic *ˀiδayn and Greek δέ: The Linguistic History of Syriac den", in Teule, H.//et. al, Syriac in Its Multi-Cultural Context (Louvain: Peeters, 2017) Pp. 13-30.
Note that there are LJLA contexts where the Aramaic lmh dyN is equivalent to Biblical Hebrew לָמָּה alone rather than לָמָּה זֶּה.
Omitted from DCPA as Syriac.