1) For securing the 'horsing' on the grindstone wheel.
When John Fyrth of Sheffield died in 1562 he bequeathed to his son his harness and smethie geare. Roger Barnsley, another Sheffield man, left his son a whele in Porters Field in 1566 and all his harness. It is likely in these cases that ‘harness’ referred to whatever secured the ‘horsing’ on a grindstone wheel.
2) Both verb and noun could be used with reference to armour or military equipment.
c.1490 with outyn that ... ony of theym wer arrayed in maner of werr ... but vj whiche were in single array and not harneyssed, Markington
1493 have theyr harnas redy as jake, salet, bowez, arowez and other weappyns, York
1539 To the said James my sone, all my harnis, Northowram
1558 to my brother Henrye Dyneley all myn aperell, harness, and weapnes, Swillington. It could also mean to decorate parts of a person’s apparel with precious metals or ornaments, girdles in particular: 1429 unum baslard harnizatum cum argento, Bramham
1449 a girdill of purpull silk harnest with golde, Southwell
1503 my best blake harnest gyrdell, Adel
1508 one small girdill hernest with sylver and gilt, Breckenbrough
1554 to Elizabeth my doughter ... a gyrdyll hernysyd wyth a pynder of siluer and a buckell, Pontefract.