An exclamation of surprise or praise. You studied hard, and, by cracky, you got an A! Isn't it funny how that works? By cracky, we've solved the mystery!
See also: cracky
Rur. By God! (A mild oath, often used to express surprise or approval.) Jim said he'd get the whole house painted this weekend, and by cracky, that's just what he did. By cracky! That thunder sure was loud.