The word "undefeatable" is correct and usable in written English.
You can use it when referring to something that cannot be overcome or someone who cannot be defeated. For example, "The champion fighter was known as the undefeatable one."
No plausible system is undefeatable.
The film opens with a montage of posters and newspaper clippings from his glory days when he was the personificastion of machismo, undefeated and apparently undefeatable in the ring.
I envision an America with a national defense unparalleled, undefeatable, and unencumbered by overseas nation building!
Mamlūk legitimacy also rested on the regime's early military successes, particularly those against the Mongols, who were seen by many contemporaries as undefeatable and as a threat to the very existence of Islam as a political culture.
In her junior year, Taurasi made a collection of inexperienced underclassmen nearly undefeatable, and the Huskies extended their unbeaten streak for 31 more games the record in women's basketball and the second-longest winning streak in N.C.A.A. history.
Kurlansky describes it as having "an almost mystical urgency": "It was neither sweet nor beautiful but it had an undefeatable power that some would call sexy, others edgy, some even said political".
You can come to think that the armies of Mordor are not just vast and scary, which they are, but limitless and undefeatable, which they aren't.
Jafar Panahi is a fifty-one-year-old Iranian film director with a restlessly intense manner and a sturdy, undefeatable sense of the absurd.
His parents still called him by his birth name, Ajai, which in Hindi means "Undefeatable".
I said, 'O.K. kids, you guys have got to be really brave little troopers, and you have to represent this spirit that is undefeatable.' I gave them something more to play off against.
As Stephen Kenneth Kelen is at pains to point out in his poem Lawn Wars, these weeds are undefeatable, and yet lawn owners feel compelled to do battle with them every year.
