Unclear.
Conceivably from Proto-Semitic *ʔišd- (“backside, lower leg, base, foundation”), and related to Classical Syriac ܐܫܬܐ (eštā, “base, bottom, buttocks”), Ugaritic 𐎌𐎚 (št), Hebrew שֵׁת (šēṯ), and Akkadian 𒁼 (išdum, “base, foundation, root”), then related to Arabic أُسّ (ʔuss) and أَسَاس (ʔasās), both meaning base or foundation.