Someone who's punic is likely to betray you. You could describe your brother as punic if he promises to keep a secret and then immediately tells it to all of his friends at school.
A punic person is treacherous or two-faced, unlikely to be loyal. You risk being called punic if you're nice to your friend but gossip about her behind her back. The adjective punic is sometimes capitalized, and in that case it means "of or relating to Carthage," which was a famous ancient empire and is still a city in Tunisia today. Because the ancient Romans thought the Carthaginians were traitorous and unreliable, punic came to also have this meaning.