Word Origin & History
O.E. eln "unit of measure of 45 inches," originally "length of the arm," from PIE *el- "elbow, forearm" (cf. Gk. olene "elbow," L. ulna, Arm. uln "shoulder," Skt. anih "part of the leg above the knee," Lith. alkune "elbow"). The exact distance varied, depending on whose arm was used as the base and whether
it was measured from the shoulder to the fingertip or the wrist: the Scot. ell was 37.2 inches, the Flem. 27 inches. L. ulna also was a unit of linear measure.
"Whereas shee tooke an inche of liberty before, tooke an ell afterwardes" .
Sense of "building extension" is Amer.Eng. 1773, for resemblance to the shape of the alphabet letter.