fucking got me, good one

It's not a loan word, it's the word for salmon in the oldest constructable ancestor of English.

Super impressive since English is only 1,500 years old...

I'm guessing you mean "Old English" since it's sometimes said to be that old, but realistically that version of English has very little in common with English now (it was verb-second, for example, like German still is today). Even the post-Danelaw version of a couple hundred years later (with Norse borrowings like "husband" and even the pronouns "they/them") resembles modern English a lot more. Middle English was largely due to the influx of Norman French (both morphological and syntactic changes), and the whole thing isn't really recognizable as quasi Modern English until around 1500-1600.

Point is: language is a continuum, and a lot of these oldest this/oldest that claims in language just have to do with where someone is arbitrarily drawing a line.

Modern German for lox is "Lachs" (same pronunciation really, and spelling ultimately doesn't matter in linguistics). This makes sense, because the English of 1500 years ago would have been relatively close to German varieties of the period. But doesn't that mean "lox/Lachs/however you want to spell it" goes back further than that, perhaps to some earlier parent of both English and German? Yes, it likely does.

Edit: and yes, as others have said, that means lox is not a borrowing (vs. e.g. "husband"). Lox existed before anyone was calling English English. But that's also true of e.g. pronoun "he" and a lot of other stuff: by definition, any word that is reconstructed in Proto-Germanic and still exists in English today is "the oldest" (but there will be many of them and they're all roughly considered to be the same age, since proto-languages are ultimately abstractions with no exact dating).

Yes that's how languages evolve. It's interesting, isn't it?

Oldest word [used] in the English language

Not oldest English word.

Many people call it lox. You can too!

For me, those are two different (but equally delicious things): https://www.foodnetwork.com/how-to/packages/food-network-essentials/lox-vs-smoked-salmon

The German word for salmon is "Lachs" but it's pronounced "Lax". I wonder who had the word first

Yes, English didn't exist 8000 years ago. Instead, there was a language called Proto-Indoeuropean spoken on the steppes of Ukraine. Just like how Latin spread and local dialects slowly became Spanish, French, Italian, Romanian, etc., PIE spread out and its descendants became Greek, Sanskrit, Russian, Latin, German, etc.

Part of what happened over time was sound shifts. For example, PIE p morphed into an f in Proto-Germanic. Father and the Latin word pater go back to the same PIE root word, but father exhibits the sound change of p -> f you saw in Germanic languages.

Similarly, Spanish has a sound change where f changed into h. So the Latin word fabulari (to chat) became hablar in Spanish and falar in Portuguese.

The point of the article is that the PIE word for salmon, laks, by random chance didn't really morph much in Germanic languages. So you have lax, lox, lachs, etc.

Interestingly, the Old English word for salmon was leax, and that made its way into Middle English and early Modern English as lax. It died out in favor of the French-derived salmon, and then we borrowed lox back from Yiddish.

It's like if beef entirely replaced cow, then we borrowed back koe or kuh from Dutch or German.

Try reading it differently.

It's a really old word (oldest) that is currently used in the English language.

Me neither lol. Ive lived my whole life in Ireland for context. I've seen and heard smoked salmon plenty of times but never lox

Ancient Sumerian refrigeration technology was seriously underrated: -297°F was no problem for them

German already calls it Lachs.

The English word comes from the Yiddish "laks," which comes from German. So while it is pronounced the same in English as it was 8,000 years ago, it was also introduced to English relatively recently, in 1934.

Important words undergo sound changes all the time.

For example, in Germanic languages, Proto Indoeuropean p sounds consistently morphed into f sounds. So the PIE word pods became Proto Germanic fots became English foot. pəter became fader became father. The preposition per became fur became for.

Lox is mostly unusual in that it didn't have any major sound changes affect it in Germanic languages.