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Umami is the taste of MSG. It's not "I'm not sure what to say it tastes like, so I'll say umsmi."
I don't think you understand your mouth as that's just bullshit. It's perfectly possible to create umami flavours without MSG, the word was coined to describe the effect of adding MSG has to flavours, but it's not like that taste was new, just poorly described before MSG was invented as an easy way to enhance that flavour.
It's because it's one of the 5 basics tastes. You can arrive at an umami flavour without adding msg.