From zarah; the spread of the fingers, i.e. A span -- span.
see HEBREW zarah
H2239. zereth
זֶ֫רֶת noun feminine span (Late Hebrew id. (זַרְתָּא ᵑ7, Syriac ; apparently from Hebrew), etymology dubious; Thes and most derive from √ זרה, see Ol§ 146 b but also Sta§ 187 a; BaZMG 1887, 607 proposes √ זרה measure off, determine, compare p. 280) — span, as standard of measurement, בַּז ׳שָׁמַיִם תִּכֵּן Isaiah 40:12; elsewhere of actual size, a span, distance between ends of thumb and little finger of spread hand; = 1/2 cubit (see אַמָּה & references; compare Exodus 25:10 with JosephAnt. iii. 6, 5), ׳ז 28:16 (twice in verse); 39:9 (twice in verse) (all P) Ezekiel 43:13; זָ֑רֶת 1 Samuel 17:4 six cubits and a span. See HultschMetrol. 1882, 443 BenzArchäol. 179.