Hapsburg jaw. Historical artwork (after Velasquez) of King Philip IV of Spain (1605-1665). Like several of his predecessors and descendants, Philip IV had a protruding lower jaw. The condition, called mandibular prognathism syndrome or 'Hapsburg jaw', was the result of inbreeding among the Austro-Hungarian monarchy. The trait was first noticed in Maximilian I (1459-1519). Philip IV's son, Charles II, had such a serious case of Hapsburg jaw that he was unable to chew. He was also mentally retarded and impotent.