dooryoudaoicibaDictYouDict[door 词源字典]
door: [OE] Old English had two closely related words for ‘door’: duru (mirrored by German tür ‘door’) and dor (which corresponds to German tor ‘gate’). They gradually came together during the Middle English period. Both go back ultimately to the Indo-European base *dhwer-, which also produced Greek thúrū ‘door’ (source of English thyroid), Latin foris ‘door’ (source of English foreign and forest) and forum, Sanskrit dvar- ‘door’, Russian dver’ ‘door’, Lithuanian dùrys ‘gate’, etc.
=> foreign, forest, thyroid[door etymology, door origin, 英语词源]
door (n.)youdaoicibaDictYouDict
Middle English merger of Old English dor (neuter; plural doru) "large door, gate," and Old English duru (fem., plural dura) "door, gate, wicket;" both from Proto-Germanic *dur- (cognates: Old Saxon duru, Old Norse dyrr, Danish dør, Old Frisian dure, Old High German turi, German Tür).

The Germanic words are from PIE *dhwer- "a doorway, a door, a gate" (cognates: Greek thyra, Latin foris, Gaulish doro "mouth," Gothic dauro "gate," Sanskrit dvárah "door, gate," Old Persian duvara- "door," Old Prussian dwaris "gate," Russian dver' "a door").

The base form is frequently in dual or plural, leading to speculation that houses of the original Indo-Europeans had doors with two swinging halves. Middle English had both dure and dor; form dore predominated by 16c., but was supplanted by door.
A door is what a dog is perpetually on the wrong side of. [Ogden Nash]