It's the early middle ages, and you and your friends are putting together your Fantasy Language Teams when a three-way deal starts to suggest itself. You're playing Old English, but your word eagðyrl, just hasn't been scoring the usage you'd hoped. You look over to Old Norse and see the word vindauga, who's languishing where he is, but you think, with a little retooling, a little training, he could have a place on your team and really become a household name.
Meanwhile, Team Norse is looking to replace vindauga with something else, but they're not interested in your eagðyrl. They're more interested in the Romance player fenestra. Fenestra's all the rage: he'll end up winning the Vocabulary League's highest prize--the Import Cup--both in France as fenêtre and in Germany as Fenster.
So what can you, Old English, give to Old High German or Old French to persuade them to send fenestra north, thereby allowing Norse to release vindauga? Well, there were many borrowings throughout history, but to pick one, let's go with Sonnabend. (You don't have to trade for the same position, after all.) The day before Sunday has several names among Germanic lands. One of the German words, Samstag comes from sabbath. If you say Samstag with a cold, you'll hear the inherent relationship between b's and m's: hence sabb[ath]'s Day > sab's Tag > Samstag.
In England, the day's dedication to pagan Saturn prevailed in the name Saturday--an irony, since the Christian missionaries to the continent preferred 'Sun-eve', or sunnanæfen, cognate of what would become the other German word for Saturday, Sonnabend. So the English word is Roman-influenced, but the German word is Old English.
<aside>Note that German did already have the Germanic roots for sun and eve. Strictly speaking, this isn't a word borrowing, but a borrowed translation. Case in point: The telephone allows you to hear things far away, hence its name from Greek tele-, far, and phoné, sound. The English word is put together from words borrowed from Greek. But German puts its word together from native Germanic roots: Fernsprecher = fern, far + sprecher, speaker. The same thing applies to Sonnabend: native (German) roots, influenced by Old English construction (sunnan + æfen > Sonn + Abend).</aside>
Anyway, with the contribution of OE sunnanæfen to German Sonnabend we can call our three-way Fantasy trade complete. Fenestra goes to Team Norse where it will become, e.g., Swedish fönster. Norse vindauga, literally 'wind-eye', comes to Old English where it will become window. OE eagðyrl is cut from the team. And Old English sends sunnanæfen to German where it becomes a household name every week as Sonnabend.
Bottom line: no one kept their original word for 'window', except possibly Old French. German and Norse took the Romance root. English has a Germanic root, but not the original Old English one. And while French kept the Romance root, it has plenty of words of Germanic origin as well (matter for another post some day).