Old English flor "floor, pavement, ground, bottom (of a lake, etc.)," from Proto-Germanic *floruz "floor" (source also of Middle Dutch and Dutch vloer, Old Norse flor "floor," Middle High German vluor "floor, flooring," German Flur "field, meadow"), from PIE *plaros "flat surface" (source also of Welsh llawr "ground"), enlarged from root *pele- (2) "flat; to spread."
Meaning "level of a house" is from 1580s. The figurative sense in legislative assemblies (1774) is in reference to the "floor" where members sit and from which they speak (as opposed to the platform). Spanish suelo "floor" is from Latin solum "bottom, ground, soil;" German Boden is cognate with English bottom (n.). Floor-plan is attested from 1794; floor-board from 1787, floor-lamp from 1886, floor-length (adj.) of dresses is from 1910. The retail store's floor-walker is attested from 1862.