"Some teach that boys should keep in the gas of their bellies by compressing their buttocks. But it is not civil to become ill while you are trying to seem polite. If it is possible to leave, let him do it alone, but if not, follow the ancient proverb: Hide the fart with a cough."
De civilitate morum puerilium (1530)
In fact it's a joke that has yet to get old. We would often find humour in farting and now it doesn't require the gas to start a chuckle. All that is needed is to clear one's throat.