"It's way too hot."
"それな。"
"Like, dangerously hot."
"それな。"
That is a complete conversation between two Japanese people under 30, and both of them enjoyed it.
Literally it is それ (that) + な (a sentence-ending particle of shared feeling). Functionally: "right?", "facts", "exactly", "I feel that in my bones".
It appeared as youth slang in the early 2010s and went nationwide fast, because it does a job that Japanese needed doing: pure agreement with zero effort and zero content. You are not adding anything. You are confirming that the other person is not alone.
That is more important than it sounds. A huge amount of Japanese conversation is 共感 (kyoukan) — showing you feel the same thing — rather than exchanging information. Japanese speakers back-channel constantly: うん, そう, へー, なるほど, ほんまに. それな is the young, flat, slightly exhausted member of that family.
Tone is everything. Fast and bright, it is enthusiastic. Slow and low, with a sigh, it means "we are both trapped in the same situation and there is nothing to be done", which is honestly its best use.
Western conversation rewards the person who adds something. Japanese conversation rewards the person who matches. If you keep offering new information and getting flat responses, you are not being disliked — you are playing tennis at a table where everyone else is passing a ball around. それな is your entry ticket to the other game.