Boarder is a noun meaning "a person who pays to live and take meals in someone's house". Historically, it is from board (in the sense 'table, daily meals provided for payment') + -er; board descends from Old English bord ('plank, table'), from Proto-Germanic *burdą ('board, plank'). The visible construction is part of the etymology: boarder is not just an unanalyzed simple word. The semantic history matters here: the earlier use is the starting point, and the modern sense, "a person who pays to live and take meals in someone's house", is the later or specialized English value. The historical chain links Old English bord with Proto-Germanic *burdą. Close English family members include board, and boarding.