köken: From Proto-Iranian, from Proto-Indo-Iranian, from Proto-Indo-European *(s)ptyēw- (“to spit, vomit”). Akin to Ossetian ту (tu, “spittle”), Sanskrit ष्ठीवति (ṣṭhīvati, “to spit”), Armenian թուք (tʻukʻ, “spit, spittle”), Serbo-Croatian pljuvati (“to spit”), Ancient Greek πτύω (ptúō, “to spit out”), Latin spuō (“to spit”), English spew.
köken: From the same stem as slieka (“earthworm”) (q.v.), with dissimilation (*sliekalas > siekalas); note dialectal forms like sliekas (“saliva”), ultimately from Proto-Indo-European *sley- (“slimy, slippery, glide”). The synonymous word slienas comes also from the same stem, but with an extra n instead of an extra k.