Origins
The word 'floor' has been in English since the language's earliest recorded period, appearing in Old English as 'flōr' with the meaning of a level surface underfoot — the bottom of a room or a paved area. It descends from Proto-Germanic *flōruz, which itself derives from the PIE root *pleh₂-, meaning 'flat,' 'broad,' or 'to spread out.'
This PIE root is one of the most productive in the Indo-European family. In Latin, it produced 'plānus' (flat, level) — source of English 'plain,' 'plane,' 'planate,' 'esplanade,' and 'explain' (literally 'to flatten out'). In Greek, it gave 'platys' (broad, flat), source of 'plate,' 'plateau,' 'platform,' 'platitude,' and 'platypus' (flat-footed). The connection between 'floor' and these Latin and Greek words is obscured by a regular Germanic consonant shift: PIE initial *pl- became *fl- in the Germanic branch. This same shift relates Latin 'plēnus' (full) to English 'full,' Latin 'plectere' (to plait) to English 'flax,' and Latin 'plōrāre' (to weep, overflow) to English 'flow.'
In the Germanic languages, cognates of 'floor' show interesting semantic divergence. Dutch 'vloer' corresponds closely to English, meaning the floor of a room. German 'Flur,' however, has shifted to mean 'hallway' or 'corridor' — a flat passage — and in its older sense 'open field' or 'meadow.' Old Norse 'flórr' was more specialized, referring specifically to the floor of a cowshed. These variations all radiate from the central concept of a flat, level surface.
Development
The original 'floor' in early dwellings was simply leveled, packed earth — the ground itself, smoothed and sometimes hardened by trampling or the application of clay. The word thus originally denoted not a constructed element but a prepared natural surface. Wooden flooring became common in wealthier European homes during the medieval period, and stone flagging in churches and public buildings even earlier, but the word 'floor' had been established long before these technologies.
The extension of 'floor' to mean 'a storey of a building' (as in 'the third floor') developed in Middle English and reflects a metonymic shift — the flat surface that separates one level from another came to stand for the entire level. This usage is now standard across English, though British and American English famously disagree on numbering: what Americans call the 'first floor' (ground level), the British call the 'ground floor,' with the British 'first floor' being one level up.
The verb 'to floor' has developed several meanings: to furnish with a floor (15th century), to knock someone down to the floor (18th century), and figuratively to astonish or confound someone (19th century, 'I was completely floored by the news'). The parliamentary expression 'to have the floor' — meaning to have the right to speak — dates from the eighteenth century and refers to the physical floor of the debating chamber, the space where the speaker stands.
Word Formation
The compound 'dance floor' reflects one of the oldest social functions of a floor: a cleared, level space for communal dancing. 'Shop floor,' 'factory floor,' and 'trading floor' extend the concept to workplaces. 'Flooring' as a collective noun for floor materials (hardwood flooring, vinyl flooring) dates from the seventeenth century, when the market for manufactured floor coverings began to develop in earnest.