That's a Dutch saying, literally translated into English. It's used in situations where somebody admits something that he or she was trying to hide and now it's uncovered. Someone else would say "Daar komt de aap uit de mouw!" = "There comes the monkey out of the sleeve!".
"Hij heeft er een broertje dood aan" - He has a dead little brother to it = to dislike something
"Hij heeft boter op zijn hoofd" - He has butter on his head = someone who criticises others, but who's also to blame himself
"Van een mug een olifant maken" - To make an elephant out of a mosquito = make a big problem out of something small
"Hij schudt het zo uit zijn mouw" - He just shakes it out of his sleeve like that = he seems to be doing it without effort
Do you have some sayings that sound funny when literally translated?