flat [ME] Flat in the sense ‘smooth and even’ is from Old Norse flatr. To flatline came into use in the 1970s for ‘to die’, from the continuous straight line displayed on a heart monitor when a heart stops beating. Flat meaning ‘apartment’ is from the early 19th century and related to the first flat. It originally meant a storey of a building and is an alteration of the obsolete Germanic word flet ‘floor, dwelling’.