An extentsion of the sexual usage of take. "To take advantage" of someone is to have sexual contact which someone who is semi or wholly unconsious due to alcohol or drug intake. The "advantage" is because they often can not decline contact or are even aware of said contact. Such practice is mostly agreed upon as morally wrong.
"I know me and Sarah have been going out for a while, but I couldn't take adavantage of her when she was wasted."