The correct expression is “뒈지게 맞기 싫으면” (되지다 is a dialect for “뒈지다”). The expression means “unless you want to be beaten to death”.
뒈지다 is a slang/vulgar expression for “죽다”(die). The word has a very negative connotation. (Be careful when you use the word; you could offend someone) It is used when someone (usually you don’t like) suffers unfortunate death or dies unhappily; for example “starve to death”; “be beaten to death”. Example: 그 도둑놈이 몰매를 맞고 뒈졌구만! (The thief has been beaten to death!)