Your site has been hacked.
While it might display normally while visiting it directly, malicious code activates when viewed through a search engine.
Usually this code resides in PHP files in a eval(base64_decode()) function.
The best way to take care of this is to go to a backup of the site which doesn't have this, and restore it. There's no telling what damage was done within your files and databases.
You're running a very old version of wordpress, so this is no big surprise. Older versions have vulnerabilities that hackers can take advantage of.