In the movie, the first few times Aguirre's name is said it sounds like 'Geary' to me. It becomes more like 'A-Geary' later...Most clearly when Jack, trying to separate the Chilean sheep from the herd he was responsible for, says 'F*** A-Gearly!'. Maybe it's a little like French--the 'a' in Aguirre gets elided with the next spoken syllable?