Etymology
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door (n.)

"movable barrier, commonly on hinges, for closing a passage into a building, room, or other enclosure," c. 1200, a Middle English merger of two Old English words, both with the general sense of "door, gate": dor (neuter; plural doru) "large door, gate," and duru (fem., plural dura) "door, gate, wicket." The difference (no longer felt in Old English) was that the former came from a singular form, the latter from a plural.

Both are from Proto-Germanic *dur-, plural *dures (source also of Old Saxon duru, Old Norse dyrr, Danish dr, Old Frisian dure, dore, dure, Old High German turi, German Tür). This is from PIE root *dhwer- "door, doorway."

Middle English had both dure and dor; the form dore predominated by 16c. but was supplanted later by door. The oldest forms of the word in IE languages frequently are dual or plural, leading to speculation that houses of the original Indo-Europeans had doors with two swinging halves.

Figurative sense of "means of opportunity or facility for" was in Old English. Phrase door to door "house to house" is from c. 1300; as an adjective, in reference to sales, by 1902.

A door is what a dog is perpetually on the wrong side of. [Ogden Nash]
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door-yard (n.)

also dooryard, "the yard about the door of a house," c. 1764, American English, from door + yard (n.1).

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door-knob (n.)

also doorknob, "the handle by which a door is opened," 1829, American English, from door + knob.

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door-knocker (n.)

"metal device fixed to the outside of a door for banging to give notice when someone desires admission," 1794, from door + knocker (n.).

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door-stop (n.)

"device placed behind a door to prevent it from being opened too widely," 1859, from door + stop (n.).

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door-strip (n.)

"border or weather-guard affixed to the edge of a door, fitting tightly against the casing when it is closed," 1849, from door + strip (n.).

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door-bell (n.)

also doorbell, "bell at a door, or connected to a knob outside a door, for the purpose of giving notice when someone desires admission," 1800, from door + bell (n.).

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door-mat (n.)

also doormat, "heavy mat placed before a door for use in cleaning the shoes by those entering," 1660s, from door + mat. Figurative use in reference to persons people "walk all over" or upon whom they (figuratively) clean their boots is by 1861.

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