taḥash

This word appears in only two settings. In the building accounts of the tabernacle it names a hide the Israelites donated and then used for the outermost layer of the tent, the skin that took the weather, and for the wrappings that protected the sacred furniture whenever Israel moved camp. Centuries later Ezekiel uses it once for the material of a pair of sandals, in a picture of a bride dressed in the finest things a king could give.

What nobody can say is what the material actually was. Hebrew already has an ordinary word for any animal's skin or hide (עוֹר), and this word sits beside it as the specific kind: "skins of tachash." That leaves two live possibilities, and scholars have argued for both. The word may name an animal, most often proposed as a sea creature of the Red Sea such as a dolphin or a dugong, whose tough hide would suit a roof. Or it may name the leather itself, a particular grade or finish of prepared hide, in which case the word was most likely borrowed from a neighboring language rather than built from a Hebrew animal name.

Two details in the text keep the question open. The material is always something of value: it is donated for the sanctuary alongside blue and purple yarn and fine linen, and in Ezekiel it belongs in a list of royal gifts. And twice the word stands on its own, with no word for skin beside it, which reads more naturally as the name of a finished product than as the name of a beast.

The ancient Greek translators of the Old Testament ran into the same wall and solved it by color, rendering the phrase as hides of a violet blue (ὑακίνθινος) rather than naming any animal. Their choice tells us how the word was being understood a few centuries before Christ. It is not a settled answer.