Valda -- with some verbs (not only this one), the prefix на- used in combination with the suffix -ся gives the general meaning "to perform the action up to or beyond the point of satisfaction".

One of my college textbooks gave the humorous example of a Russian grandmother who's encouraging her little grandson to have yet another serving of macaroni -- "Mangia, mangia, you're all skin and bones," as an Italian grandma would say -- and the kid complains with a word of his own invention:

"Бабушка, я уже намакаронился!!" (Roughly, "Grandma, I'm already all-macaroni'd-out!")

However, be aware that the на-/-ся combination does NOT have this meaning for all verbs.