The Oxford English Dictionary defines "shiver my timbers" as "a mock oath attributed in comic fiction to sailors." ( source ) But other sources say it is more literal - that a shiver back in the day indicated something breaking into pieces. So when a cannonball would hit it would shiver the timbers. ( source ) It seems today it's used mostly to talk like a pirate, ha! But it is also used to express surprise - both good and bad.