(Anglo-Saxon eln, an ell). It is said that the English
ell was the length of Henry I.'s arm, but the ordinary length of a
man's arm is about a yard.
Give him an inch, and he'll take an ell.
Give him a little licence, and he will take great liberties, or
make great encroachments. The ell was no definite length. The English
ell was 45 inches, the Scotch ell only 37 inches, while the Flemish ell
was three-quarters of a yard and a French ell a yard and a half. This
indefinite measure expresses the uncertainty of the length to which
persons will go to whom you give the inch of liberty. Some will go the
French ell; while others of more modesty or more limited desires will
be satisfied with the shorter measures.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894