A Slughorn is pretty much an undesirable woman with a loud mouth. Whether she is fat, ugly or a straight up retard, she must be undesirable.
However, in some circumstances a desirable woman may be called a slughorn. For example during times of drunken frustration or when she has just pissed you off.
When that drunken fat chick at the footy wouldn't shut the fuck up! Leon had no choice but to call her a Slughorn.
" You have too slay a few Slughorns to get to the Princess."