Etymology
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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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knob (n.)
late 14c., knobe, probably from a Scandinavian or German source (compare Middle Low German knobbe "knob," Middle Dutch cnoppe, Dutch knop, Old Frisian knopp, knapp, Old High German knopf, German Knopf "button," Old Norse knyfill "short horn"). Meaning "knoll, isolated round hill" is first recorded 1640s, especially in U.S. For pronunciation, see kn-.
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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-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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next-door (adv.)

also nextdoor, "in or at the next house," 1570s, from noun phrase next door "nearest or adjoining house" (late 15c.), from next + door. As an adjective from 1660s. Noun meaning "the people living next door" is from 1855. Middle English dwellen at dores (late 14c.) meant "live next door."

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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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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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