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The Greeks and Romans regarded them as the attire of barbarians (including the Celts, you know ),
Very true; that's why traditional Clergy still wear robes after 2,000 years. It was a sign of being 'civilized'. Trousers were frowned upon. They were mainly for the colder northern climates. Kilts, of course were woollen plus tall hose, so managed quite well in the Highlands with the added comfort of freedom of movement. Plus in wading icy streams, there were no wet trouser legs to cause discomfort- just lift & wade.