Someone who has been 'holding in' a shit for a period of time, thus allowing the faeces to 'cook' or 'bake' inside their anus.
Usually occurring through lack of toilet facilities although somebakers have been known to bake for pleasure,fame or purely just the for the craic.
Dave:"You taking a dump later?"
Joe:"Sure am, I've been baking it for over two days now"
Dave:"lol I hope it hurts you filthy shit baker"