From an alteration (due to elder) of Middle English ellern (“eldern”), from Old English ellærn, ellarn (“of elder-wood, eldern”), equivalent to elder + -en.
Meanings
(not-comparable, obsolete) Made of elder wood, or of the elder tree.
[T]ruth, a heauen he would diſcharge vs as boyes do elderne guns, one pellet to ſtrike out another: […] c. 1603 (date written), Iohn Marston, The Malcontent, London: […] V[alentine] S[immes] for William Aspley, […], published 1604, →OCLC, Act IV, scene iv: