The Collaborative International Dictionary of English v.0.48:

Adlegation \Ad`le*ga"tion\, n. [L. adlegatio, allegatio, a
   sending away; fr. adlegare, allegare, to send away with a
   commission; ad in addition + legare to send as ambassador.
   Cf. Allegation.]
   A right formerly claimed by the states of the German Empire
   of joining their own ministers with those of the emperor in
   public treaties and negotiations to the common interest of
   the empire. --Encyc. Brit.
   [1913 Webster]