What is the X in X-mas?
Old English flor "floor, pavement, ground, bottom (of a lake, etc.)," from Proto-Germanic *floruz "floor" (cf. Middle Dutch and Dutch vloer, Old Norse flor "floor," Middle High German vluor, German Flur "field, meadow"), from PIE *plaros "flat surface" (cf. Welsh llawr "ground"), enlarged from *pele- (2) "flat, to spread" (see plane (n.1)).
Meaning "level of a house" is from 1580s. The figurative sense in legislative assemblies (as opposed to the platform) is first recorded 1774. Spanish suelo "floor" is from Latin solum "bottom, ground, soil;" German Boden is cognate with English bottom. Floor plan attested from 1867.