Another scource gives: the word appeared first in English litterature, somewhere before 1010. I think the Emperor Charles V could be called a Suzerain. He was overlord, Emperor of a good number of 'domains':Spain,Austria, Germany, the Netherlands,meaning Holland and Belgium at that time.Maybe even some city states in Italy?
It was him and his son Philip II 'we' fought in the 80-years'war and got them out.
The word has a French ring to it :souverain. Could it have to do with soumettre ?- to subject to, or with sauver- save. Offering protection in exchange for fealty? Yes,I've always like that word too,suzerain, it makes me think of raspberry flavored candy.A sour-sweet word.
(could it be applied to any present ruler really?)(not the candy!)