I like that song, but if you’re like me, the definition of “wassailing” may have just temporarily slipped your mind. Wikipedia says:
“The practice has its roots in the middle ages as a reciprocal exchange between the feudal lords and their peasants as a form of recipient initiated charitable giving, to be distinguished from begging….
Although wassailing is often described in innocuous and sometimes nostalgic terms, the practice in England has not always been considered so innocent. Wassailing was associated with rowdy bands of young men who would enter the homes of wealthy neighbours and demand free food and drink in a trick-or-treat fashion. If the householder refused, he was usually cursed, and occasionally his house was vandalized.”
So, it’s basically going around making noise, demanding free stuff, getting drunk, and messing things up.