*el- Look up *el- at Dictionary.com
Proto-Indo-European root meaning "elbow, forearm." It forms all or part of: elbow; ell (n.1) unit of measure; uilleann; ulna.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit anih "part of the leg above the knee;" Greek olene "elbow;" Latin ulna, Armenian uln "shoulder;" Lithuanian alkune "elbow;" Old English eln "forearm."