The first time during a long drinking session that one has to finally answer the call of nature or take a piss. This is a bad thing as this generally signals that there will be the beginning of many frequent drink-interupting trips to the dunny.
I held on as long as I could but after five beers I had to take the fatal first piss.
The short name for a chemical known as methyl mercaptan which is a naturally-occuring but toxic, smelly gas. Its chemical formula is CH4S which is very similar to methane (CH4). Both of these gases are found in farts and the mercaptan is responsible for the stench associated with flatulent activities.
I am paying for eating those boiled eggs for breakfast. I am passing mercaptan and methane with gay abandon.