next-door adv.

"nearest in place, position, rank, or turn," Middle English nexte, from Old English niehsta, nyhsta (West Saxon), nesta (Anglian) "nearest in position or distance, closest in kinship," superlative of neah (West Saxon), neh (Anglian) "nigh;" from Proto-Germanic *nekh- "near" + superlative suffix *-istaz. Cognate with Old Norse næstr, Dutch naast "next," Old High German nahisto "neighbor," German nächst "next."

In reference to time by c. 1200. Adverbial ("next to, immediately after; almost, within a little of") and prepositional ("nearest to, immediately adjacent to") uses are from c. 1200. Phrase the next man "a typical person" is from 1857. Next-best "second best" is by 1670s.

"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]