What is むり “Muri” (In Chinese characters: 無理）?
As you may know, “desu (です）” can be added to make a Japanese sentence sound polite. So in this case as well, “Muri desu （むりです）” sounds better. But it can be used without “desu” if you want to say it casually. “Muri” is used when you think it is impossible to do something. For example, a friend of yours asks you to lend him one million dollars, you'd probably reply, “Muri!” Or if your boss asks you to finish some difficult and time-consuming task by tomorrow, you'd probably want to say, “Muri desu” instead of just “Muri!” “Muri” always shows a refusal, but if you say to your coworker, “Muri shinaide （むりしないで）,” it means “Don’t work so hard.” It is a good expression since it shows your companionship with each other. Use it when you see your coworker always busy working.
As a side note, the Korean word for "muri" is "무리" which is pronounced the same way. Korean, like Japanese, has words and phrases derived from Chinese characters. Most of them have the same pronunciation as in Japanese. This word "muri (無理)" is one of them.
There are similar words: "Mura" and "Muda." See the relate article here to see the difference between them.
<Some useful expressions using "Muri,むり,無理" with English translation>
What he says is “unreasonable” (“against reason”).
Such “an unreasonable” (“unjustifiable”) “thing” can't be accepted.
“It's natural” that he quit the club.≒He “had a good reason” to quit the club.
She always makes “unreasonable” requests.
It's “impossible” for me to read all these books in a week.≒I “cannot” read all these books in a week.
You don't have to “force yourself.”
Please don't “work too hard.”
Am I asking you “too much?”
It “may not be possible” right now, but someday I'd like to own my own store.
I really think you also should go there sometime. Of course, I'm not saying “you should go against your will,” however.