A lowlander is a person who lives in, or comes from, the lowlands — the lower, flatter country, not the mountains. In Britain, Lowlander often means someone from the Lowlands of Scotland, set against a Highlander. The ending -er is the same kind of ending as in New Yorker: it names a person from a place, not the place itself. The land is lowland or the lowlands; the person is a lowlander. Outside talk about Scotland it can sound a little bookish.