“door” is an ancient word. It has cognates in other Germanic languages. Dutch deur; German Tür; Yiddish טיר (tir); Danish/Norwegian dør; Swedish dörr; Gothic 𐌳𐌰𐌿𐍂 (daur); Icelandic/Faroese dyr - this last is plural, and we’ll get back to that.
Let’s also note German Tor, as opposed to Tür, which means “gateway” and has drifted to mean “goal” in sports, both the target and the score. Proto-Germanic had two words, *durz for doors and *durą for a BIG door. In most descendant languages they merged, but not in German.
The Proto-Indo-European root is *dʰwer- and it has many descendants meaning “door”: Welsh dôr/drws, Breton dor, Irish doras; Russian дверь (dver’); Albanian derë; Armenian դուռ (duṙ); Sanskrit द्वार् (dvā́r), Ossetian дуар (duar); Latin foris, Greek θύρα (thýra).
Going further: Old Persian 𐎯𐎺𐎼𐎹𐎠 (duvarayā); Farsi/Dari در (dar), Tajik дар (dar); Urdu دوار, Hindi द्वार (dvār); Marathi दार (dār); Bengali দ্বার (dbāra); Telugu ద్వారము (dvāramu); Gujarati બારણું (bārṇũ); Burmese ဒွာရ (dwara); Thai ทวาร (spelt dwār, pronounced tá-waan).
Icelandic and Faroese dyr are always plural. So are Latvian durvis, Lithuanian duris; Belarusian дзверы (dzvjéry), Ukrainian двері (dvéri), Czech dveře, Slovak dvere, Polish drzwi. This suggests that ancient tribes had double doors at the entry to the compound.
There are interesting cases of shifting meanings. In many Slavic languages, words for “courtyard”, “court”, “palace” come from this root - Slovak & Serbo-Croat dvor, Czech dvůr, Russian, Bulgarian & Macedonian двор (dvor), Ukrainian двір (dvir). Lithuanian dvaras means “estate”.
In Latin, *dʰw -> f and forās is “outdoors”. You go out to the *forum*, a public place. The evidence suitable for public examination is *forensic*. In medieval French, people from outside are forain, which becomes English *foreign*, and the wild places outside are the *forest*.
And here’s another spin: from Greek θύρα (thýra), door, comes the word θῠρεός (thýreos), meanings include an oblong shield, adjective θυρεοειδής (thyreoidés) is applied to the shield-shaped cartilage of the larynx, which then gives its name to the adjacent *thyroid gland*.
Finally, a whole language takes its name from this root. The Persian word دربار (darbār) means a court or court gathering (cf Slavic двор/dvor above, and English durbar). The Persian spoken at court became known as دری Dari, as opposed to the فارسی Farsi spoken elsewhere.
So there you are. The ancient Indo-European root *dʰwer- gives us “door”, “forum”, “forensic”, “foreign”, “forest”, “thyroid”, “Durbar” and the name of the Dari language. Not bad for a simple mechanism to keep wild animals out and tame animals (and humans) in.