Sirius stared around at the students milling over the grass, looking rather haughty and bored, but very handsomely so.
Harry Potter and the Order of the Phoenix
I feel "very handsomely so" here means in a very handsome way. But I don't know if my understanding is correct.
Yes, in that case it is the usual meaning of handsome and the usual meaning of forming an adverb from an adjective with that suffix.
Just be careful in future, as handsomely is often used about a different sense of handsome, as in "I will reward you handsomely". It's still "in a handsome manner", but the meaning of handsome is different.
