means A main public road, especially one built for fast through-traffic between towns and cities.

from Plainly literal Old English: 'heah' (high) plus 'weg' (way). The 'high' didn't mean elevated in importance only — many old roads were quite literally built up on raised banks of earth and gravel to stay above the mud and flooding, making them the 'high ways' as opposed to the lowly tracks beside them. The word has carried the sense of 'principal public road' for over a thousand years, long before anyone merged onto anything.