door (n.)

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- (source also of Old Saxon duru, Old Norse dyrr, Danish dør, Old Frisian dore, dure, Old High German turi, German Tür), from PIE root *dhwer- "door, doorway." 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]