"unit of measure of 45 inches," Old English eln, originally "forearm, length of the arm" (as a measure, anywhere from a foot and a half to two feet), from PIE *el- (1) "elbow, forearm" (cf. Greek olene "elbow," Latin ulna, Armenian uln "shoulder," Sanskrit anih "part of the leg above the knee," Lithuanian 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 Scottish ell was 37.2 inches, the Flemish 27 inches. Latin ulna also was a unit of linear measure, and cf. cubit.
Whereas shee tooke an inche of liberty before, tooke an ell afterwardes [Humfrey Gifford, "A Posie of Gilloflowers," 1580].
type of building extension, 1773, American English; so called for resemblance to the shape of the alphabet letter.