Wiktionary
n. (context euphemistic English) A scatological or vulgar phrase intended to be amusing.
Wikipedia
Toilet humour, or scatological humour, is a type of off-colour humour dealing with defecation, urination, and flatulence, and to a lesser extent vomiting and other body functions. It sees substantial crossover with sexual humour, such as dick jokes.
Toilet humour is popular among a wide range of ages, but is especially popular with children and teenagers, for whom cultural taboos related to acknowledgement of waste excretion still have a degree of novelty. The humour comes from the rejection of such taboos, and is a part of modern culture. Examples can also be found in earlier literature, including The Canterbury Tales by Geoffrey Chaucer.