Okay, kiddo, so you know how sometimes people do certain things or say certain words because they think it's the right way even though it's actually not? Well, that's a bit like mumpsimus.
Mumpsimus is when someone does something the same way for a really long time, even though it's actually incorrect or outdated. It's like they're stuck in their ways and don't want to change, even if it would be better to do things differently.
For example, let's say your grandpa always tells you to put a stamp on a letter by licking it and then sticking it on the envelope. But now we have self-adhesive stamps that don't need to be licked. If your grandpa keeps doing it his way and won't even try the new way, that's like mumpsimus.
It's important to be open to new ideas and ways of doing things, even if it means changing our habits. Mumpsimus can prevent us from growing and learning, so it's best to try to avoid it as much as we can.