The German vocabulary word "eitel" nowadays means "conceited", but also "mere". In the Middle Ages it was used in the sense of "pure", and this is the original meaning of the name. Later (beginning at the end of the 14th century) it was used together with another forename, now meaning "only". So Eitelfritz would mean that the bearer of this name had only one given name (Fritz).
(This is taken from; Rosa und Volker Kohlheim, Das große Vornamenlexikon, Mannheim 2003)