If something is hostile, it's unfriendly. If you are zealous about the political causes you believe in, you will be hostile to any suggestion that the other side of the argument is reasonable.
Hostile comes from the Latin word hostis, for enemy, and you can hear its relation to the word hostage. But you don't have to be from a warring nation to be hostile. You can talk about a hostile nation, a hostile takeover, a hostile remark, or a hostile attitude. Some synonyms are inimical, antagonistic, unfavorable, unfriendly.