He learned this in high school, in Jordan, likely from one of his teachers. When you have hope that somebody would do something right/good, for a change, but they do not, this would be the time to say it.
Original Script: دنب الكلب اعوج. لو بنحط بميت قالب بضل اعوج
Transliteration: Danab el kalb awaj. Law binhat bi meet aleb bidal awaj.
Literal Translation: The dog’s tail is curved. Even if you put it in 100 molds stays curved.
Smooth Translation: The dog’s tail is curved. Even if you put it in 100 molds, it stays curved.
I found this proverb particularly funny because we have a pug, and his tail is always curled, so the imagery was clear. This is similar to the English proverb “A leopard never changes its spots,” in that both of them use immutable characteristics of animals as a comparison to someone who cannot change. By using the hyperbole of 100 molds, the proverb uses a bit of humor to take the edge off of the indirect advice/criticism.