1 A habitual gesture or way of speaking or behaving.
1.1 Psychiatry An ordinary gesture or expression that becomes abnormal through exaggeration or repetition.
2 mass noun – Excessive use of a distinctive style in art, literature, or music.
3 (also Mannerism)
mass noun A style of 16th-century Italian art preceding the Baroque, characterized by distortions in scale and perspective and the use of bright, often lurid colours. It is particularly associated with the work of Parmigianino, Pontormo, Vasari, and the later Michelangelo.
Bob was often accused of expressing himself with a mannerism, perhaps in a passive/aggressive way, through an exaggerated shrug.