Whisper is the ability to send a private chat message to another pirate.
How To Whisper
To send a whisper, click directly on the pirate himself, or, if not visible on-screen, click on his name (either from the Hearties menu or even from the Chat Box).
The pirate's detailed information will appear. Click on the Ear Icon. The text "Whisper To" will appear in the chat box. Any text entered and sent will go ONLY to that pirate.
Be careful who you whisper what to. The chat box will remember the last pirate you whispered to - so make sure it's the right person before sending something you don't want the wrong pirate to hear.