The phrase BC can refer to many different things, but most likely, in the case of text language, it means "because." Other common uses of the phrase BC can mean "birth control" or "be careful."
There are less likely phrases that BC could refer to, such as Bill Clinton, Boston Common, beauty contest, and base camp. The use of chat and Internet language depends primarily on the context in which it was used.
Other common Internet and chat acronyms include ASL, which is a question asking a person's age, sex and location, IDK, AFAIK, TBH and SMH. IDK typically stands for "I don't know," AFAIK means "as far as I know," TBH stands for "to be honest," and SMH means "shaking my head." With the dawn of smartphones, some text shortcuts are used less frequently, but ones such as BC and IDK remain common in texts and chats.
Generally speaking, when the letters BC are put together, especially after a date, it will mean "Before Christ." Before Christ is a Christian calendar concept that refers to the time period on history before Christ was born. In more recent years, Before Christ has been used less frequently in favor of the term BCE, which stands for "Before Common Era."