A cat's tongue is for some reason associated with an aversion to hot foods. I guess I haven't seen a cat drinking up boiling milk, but whatever. Just imagine a cat licking up something super hot and freaking out. Cats don't like hot things, which is why this is an aversion to hot foods.

The reading is the two vocab words separately put together, just as you'd expect from a body-part related kanji. So, 猫 plus 舌 = ねこじた. Just remember that 舌 is read here as じた, and not した.