Probably from shamem (in the sense of poisoning); a lizard (from the superstition of its noxiousness) -- spider.
see HEBREW shamem
H8079. semamith
שְׂמָמִית noun feminine a kind of lizard (so ᵑ7 ᵑ9 ᵐ5Leviticus 11:30 compare StrPr (also on שׂ > שׁ)); — absolute ׳שׂ Proverbs 30:28 (compare Shipley-CookEncy. Bib. LIZARD).
שׂמר (apparently √ of following; meaning dubious).