Euphemism describing the act of defecating, metaphorically speaking. Otherwise known as taking a good hard shyte for oneself. This particular expression is used when the excrement is of a particularly large and solid nature.
'A thing of' is a very Irish way of describing an inanimate object, usually a container or packaging of sorts. Be it down to laziness, or actually not knowing the name of what the object is, this expression has many uses.