Apparently masculine plural construction of esheth in the sense of an afterthought (used only in connection with asar in lieu of 'echad) eleven or (ordinal) eleventh -- + eleven(-th).
KJV: The length of one curtain shall be thirty cubits, and the breadth of one curtain four cubits: and the eleven curtains shall be all of one measure.
NASB: "The length of each curtain shall be thirty cubits, and the width of each curtain four cubits; the eleven curtains shall have the same measurements.