the.com/flatness

means The quality of being flat — lacking depth, variation, texture, or in a more figurative sense, having no emotional charge, liveliness, or interesting contour at all.

from Straightforward stuff: 'flat' traces back to Old Norse 'flatr,' meaning level or smooth, which itself descends from a much older Germanic root tied to 'flattening' or 'spreading out.' English borrowed the word around the medieval period and simply tacked on '-ness' — the standard suffix for turning an adjective into the abstract noun of itself. No twist, no myth: it has meant 'the state of being flat' for about as long as 'flat' has meant flat.