As in any other ordinary family, even the royals used affectionate names for each other (nicknames). Prince William and Kate are not only no exception, but are the cutest of all members when it comes to nicknames. 

What does the princess fondly call her husband? "Big Willy" - "Big Willy"! It seems a little strange that the *so serious* prince has such a funny nickname, but if for Kate it's flirty and sexy, everything is fine. 

Anyway, it seems Middleton doesn't use "Big Willy" anymore because she can't use it out of place (protocol), but the nickname dates back to her college days. Even during the royal wedding, when William and Kate's guests learned the nickname from Harry, who used it during his speech at the ceremony. 

Prince Harry tried to call his brother by Kate's favorite name, but "Big Willy" only sounded good from her!

Source: Cosmopolitan