Ancient measure based on the length of the forearm (5)

I believe the answer is:

cubit

(Other definitions for cubit that I've seen before include "Biblical length of approximately a forearm" , "Ancient unit of length based on the length of the forearm" , "about eighteen inches" , "Elbow-to-finger measure" , "Ancient measure about the length of an arm" .)

I've seen this clue in The Mirror.

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