WordNet sense 1 (the eleventh month of the civil year; the fifth month of the ecclesiastical year in the Jewish calendar (in July and August)):

Av

WordNet sense 2 (a bachelor's degree in arts and sciences):

BA, Bachelor of Arts, Artium Baccalaurens

WordNet sense 3 (the muscles of the abdomen):

abdominal, abdominal muscle

WordNet sense 4 (the blood group whose red cells carry both the A and B antigens):

group AB, type AB