1) A bent piece of iron, inserted to strengthen a joint (OED). The following much earlier references are in a sequence in which the jointers are used in pairs, for cupboards and windows, linked with hasps, staples, and bands.
1419 iij seris et j pari juncturarum ... 4s 8d , York
1519 Item paid for a payr of jontterres to a ambre in the Kyrk, ijd
1535 Item for hespys and stapylles and jontters to ys window, iiijd
1548 Item for two payre of juncters to his halle wyndoys , York. In 1595 John Petche was paid for two pair of Jointed irone bands for the north ille dowre of Howden Church.