From PIE *pleh₂- (flat, to spread) — the same root behind Latin 'plānus,' linking English 'floor' to 'plain' and 'plane.'
The lower surface of a room on which one stands; also, a level or storey of a building.
From Old English 'flōr' (the floor or ground of a room, a flat surface underfoot), from Proto-Germanic *flōruz (flat ground, floor), from PIE *pleh₂- (flat, broad, to spread flat). The same root underlies Latin 'plānus' (flat, level — source of 'plain,' 'plan,' 'plane,' 'explain'), 'plānēs' (flat surface), Greek 'platýs' (broad, wide — source of 'platypus,' 'plateau,' 'plaza,' and 'place'), Welsh 'llawr' (floor, ground), and Irish 'lár' (floor, middle). The Proto-Germanic *flōruz is a suffixed o-grade form meaning essentially 'the flat thing' or 'the spread-out surface.' In a world before wooden or stone flooring, the floor of a dwelling was simply the levelled
English 'floor' and 'plain' are distant cousins, both from PIE *pleh₂- (flat, broad). The initial 'pl-' of the PIE root became 'fl-' in Germanic through a regular sound shift — the same pattern that links Latin 'plēnus' (full) to English 'full' and Latin 'plōrāre' (to weep) to English 'flow.'