Well, the obvious conclusion would be that this is a reference to the historical fact that the lowlands tended to have a lot more non-Gaelic speakers living in them.
Of course, place names can be tricky, so it isn't always wise to go with the obvious. They tend to be very "conservative", in that they can be the oldest words in use in a language, or even predate the language itself. Often times new settlers in an area will borrow an existing place name from the existing residents, and then bash it into some kind of slightly different form that is easier for them to pronounce, or has some plausible local meaning. So, in the absence of any evidence to its origins, it would be quite possible for it to have been an old Pictish word that the Gaels(Scotts) took up when they took over the highlands sometime around the 10th century, and then slightly transformed so that it actually meant something in Gaelic.