arbour: [14] Despite its formal resemblance to, and semantic connections with, Latin arbor ‘tree’, arbour is not etymologically related to it. In fact, its nearest English relative is herb. When it first came into English it was erber, which meant ‘lawn’ or ‘herb/flower garden’. This was borrowed, via Anglo-Norman, from Old French erbier, a derivative of erbe ‘herb’.
This in turn goes back to Latin herba ‘grass, herb’ (in the 16th century a spelling with initial h was common in England). Gradually, it seems that the sense ‘grassy plot’ evolved to ‘separate, secluded nook in a garden’; at first, the characteristic feature of such shady retreats was their patch of grass, but their seclusion was achieved by surrounding trees or bushes, and eventually the criterion for an arbour shifted to ‘being shaded by trees’.
Training on a trellis soon followed, and the modern arbour as ‘bower’ was born. The shift from grass and herbaceous plants to trees no doubt prompted the alteration in spelling from erber to arbour, after Latin arbor; this happened in the 15th and 16th centuries. => herb
