relativity of comedy norms

In his book The Gate Francois Bizot describes how, when he was being held prisoner by the Khmer Rouge, one of his fellow prisoners used to bring him a piece of bay kdang, 'the crust of rice that had been hardened and burnt during cooking', and how he made this man 'smile shyly...by mechanically reciting an adage well known in country areas: Bay kdang, reug kda'. He translates this as 'Crust of rice, stiff penis.' Maybe this only becomes a joke in context - 'you had to be there'; it's an adage so might resemble 'crust of rice will put hairs on your chest'. An adage might be similar to a proverb, and some Cambodian proverbs also read oddly. David Chandler, in his The Land and People of Cambodia quotes about ten, including 'Don't believe the sky; don't believe the stars' and 'If a tiger lies down, don't say, "The tiger is showing respect" '.