וַעֲשֵׂ֤ה לָהֶם֙ מִכְנְסֵי־בָ֔ד לְכַסּ֖וֹת בְּשַׂ֣ר עֶרְוָ֑ה מִמָּתְנַ֥יִם וְעַד־יְרֵכַ֖יִם יִהְיֽוּ׃
And thou shalt make them linen breeches to cover the flesh of their nakedness; from the loins even unto the thighs they shall reach.
Read kabbalistic (mystical) interpretation of Exodus 28:42: and classical mystical sources.