Ich glaube mein Schwein pfeift.
This well-known (and well-worn) German expression literally translates to “I think my pig is whistling!” and is used to exclaim disbelief, usually at an absurd or extremely unexpected situation. Although every German is familiar with this idiom, it’s actually not used very often.
Opinions also diverge on where this odd phrase comes from — although its very oddness suggests that it is essentially an absurd nonsense situation that would create the reaction of disbelief because pigs apparently don’t whistle (although, for all pig-owners: If your pig indeed produces whistling of wheezing noises, it could be a sign of lungworms, which are no laughing matter!).
Ich glaube mein Schwein pfeift! Ich hätte nie gedacht, dass er so alt ist. I don‘t believe it! I never would have thought that he‘s so old!