Japanese Mispronunciations/Sun Deer
Appearance

This is a Joeism

When Joe says this, it's not a reference to any stream. It's just something he says sometimes.
"Tsundere", (ツンデレ, pronounced [t͡sɯndeɾe] [Soon-dare-ay]) is a Japanese word mainly used to describe a standoffish-but-warming character archetype in Japanese fiction, which Joe pronounces as "sun deer" instead.[1]
References
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“Joseph Anderson Channel Two” video: “Undertale First Blind Full Playthrough” at 00:56:43.