Why does fourscore means eighty?

"Fourscore" means eighty because "score" is an old term that refers to twenty. Therefore, when you multiply four (the "four" in "fourscore") by twenty (the value of a score), you get eighty (4 x 20 = 80). This usage is famously known from Abraham Lincoln's Gettysburg Address, where he begins with "Four score and seven years ago," meaning 87 years ago.