A buckmuck is them morning after poo from the previous nites bottle of buckfast (see definition of buckfast and buckie). It is often an extemely traumatic experience but is always neccessary. Having consumed over 400 bottles of buckfast personally, I have yet to recall even one instance where a buckmuck was not required the following morning (on rare occasions,they may be required even before the bottle has been fully consumed). They are usually black as the ace of spades and should only be taken with a window wide open.
"were you drinking buckfast last nite"
"oh yea, I had a serious buckmuck this morning. Black as coal they were"