ell a former measure of length (equivalent to six hand breadths) used mainly for textiles, locally variable but typically about 45 inches in England and 37 inches in Scotland.

The word is recorded from Old English (in form eln) and is of Germanic origin, from an Indo-European root shared by Latin ulna (‘humerus’). Like the cubit, the measure was originally linked to the length of the human arm or forearm.
give someone an inch and they will take an ell undue advantage will be taken of a slight concession (saying recorded from the mid 17th century).

See also a miss is as good as a mile.

ell measure of length (in England 45 inches). OE. eln = MDu. elne, elle (Du. el), OHG. elina (G. elle), ON. ǫln (aln-) cubit, ell, forearm, Goth. aleina cubit :- Gmc. *alīnǒ, orig. ‘(fore)arm’; cogn. with L. ULNA, Gr. ōlénē, ōlḗn ELBOW, OIr. u(i)len, W. elin, Skr. aratní-, etc.

ell.
1. Measure of length, in England once 45 inches.

2. Extension or wing added at right angles to the principal direction of the original building in the USA.