א֤וֹ בִֽשְׁתִי֙ א֣וֹ בְעֵ֔רֶב לַפִּשְׁתִּ֖ים וְלַצָּ֑מֶר א֣וֹ בְע֔וֹר א֖וֹ בְּכָל־מְלֶ֥אכֶת עֽוֹר׃
or in the warp, or in the woof, whether they be of linen, or of wool; or in a skin, or in any thing made of skin.
Siftei Chakhamim
Or of wool. The ל is like the ל of (Bereishis 32:18): “למי אתה” which means: To whom do you belong?
Chizkuni
.או בשתי, “or in the warp;” the word שתי is not a genitive, construct mode, but a word in its own right, meaning “foundation”. Compare Psalms 11,3: השתות, “the foundations;” the reason is that the warp is the basis of all manner of weaving.