This slang is not so used. It means "incredbly". As I know, this slang comes from "de morrer". That was "Linda de morrer'. As it was associated with death, people started to switch "de morrer' by "de viver"

Man,at least in brazil this slang isn't used at all,but it means "very" or "a lot",like,she is very beautiful

Hi Jared, "De viver" is a brazilian old slang, we don't use that anymore. Usually, you can use another words to express something impressive. For example: amazing (She's amazing - Ela é maravilhosa), gorgeous (She's gorgeous - Ela é deslumbrante) and etc.