There are, impressively, many words in Turkic languages and dialects that survive as remnants of Old Turkic - but teyeñ or teyin is by far the one that surprised me the most. 🐿

Happy World Turkic Language Family Day!
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“Yes! Where did you hear that from?”

He replied, “That’s what it used to be called by the elders back in the old days. Young people nowadays don’t know it.”

Teyeñ is an Old Turkic word for squirrel (or marmot?) attested in the Orkhon inscriptions from 735 CE as kök teyeñ (𐰚𐰇𐰚:𐱅𐰘𐰭) - gray squirrel.

I showed my father-in-law a short funny video clip of a squirrel. He responded in Turkish:
“Ne o? He. Teyin. Sincap.” (What is that? Oh. Teyin. Squirrel.)

My eyes lit up;
I asked, “What did you just call it?”
He looked confused: “Sincap?”
I said, “No! What did you call it before that?”
“Teyin?”