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Literally nobody asked but i feel the need to explain.
There are 6 named Avari peoples, much like the Noldor, Vanyar and Sindar. The one that lives the furthest north are the Hwenti. Before it's destruction at the end of the first age, the Hwenti have a duty to guard Middle Earth against the things that crawl out of ruined Utumno.
Erestor was not born into the Hwenti people but is absolutely considered one of them once he takes up this duty. He's part of the branch that defends the Western Arctic of First Age Middle Earth... right near the Helcaraxë.
One fine... well the sun hasn't risen yet so it's not a day... anyway, he leaves the camp to keep watch for wargs and catch some fish.
He returns completely baffled with 2,000 starving doomed Noldor and absolutely no idea who they are or what they're doing here, except that they maybe escaped the Far God's Land?? But don't seem to know how to make functional coats or navigate the ice floes??
His unflappable CO, bless her heart and patience, goes "well, they aren't fish, that's for damn sure" and it catches on!
Fingolfin's Noldor are hitherto jokingly referred to as the Fish People - a gag which grows in infamy as the Noldor proceed to set fire to Beleriand - and with the joke goes the story of the poor Hwenti fuck who went out to get fish and came back with the Tyrants of the West. Poor guy. Poor, stupid guy. What a legend.
It becomes a general catch-all phrase for stupid shenanigans and stupid people: like kitchen maid no. 1 goes "hey, did you hear that the Empress' second son is secretly engaged to the rebel leader trying to depose her??? It's scandalous!!" and kitchen maid no. 2 goes "wow! and do you also believe that the Noldor are fish?? because that's absolute bullshit!" - and general 1. says to general 2. "we could attempt that defensive manoeuvre, if we felt like catching Noldor instead of fish and causing a massacre instead of covering our retreat" - tailor unimpressed by assistant acquiring 100 bolts of pomegranate satin when he specifically requested carmine silk; assistant defends herself by saying "well at least it's not 100 Noldor - we can still make this work!" - Silvan soldier at the Last Alliance, with a grand gesture towards the Noldorin Armies, lisps "look at all those fishes!"
Anyways, Legolas meets Erestor and sure it's cool and all that he's Lord Elrond's Chief Councellor but uh. The Silvan are a cultural fusion of Sindar and Penni (another Avari group) so of course he's heard the Noldor are Fish Gag, and of course he knows all about the Hwenti guy who went out for fish and came back with the Doomed Hosts of the Noldor. It's like if you met the 'What The Fuck Richard' guy and he's somehow the Vice President of Switzerland sending you on a top secret mission to destroy Russia's nuke codes?? Like ok sure this is important but consider: I Know What You Are
Elrond: And this is my Chief Councellor, Erestor. He is 8000 years old, and very wise and venerable. I trust him with my life. Legolas, Silvan, knows all the shit that his guy did: Oh, the Fish Guy! Hey! ヾ(＾ ∇ ＾) Erestor, the Fish Guy: ... hello