Definition of "placidity"

And how could she bear such behaviour! Composure with a witness! to look on, while repeated attentions were offering to another woman, before her face, and not resent it.—That is a degree of placidity, which I can neither comprehend nor respect.

1815, Jane Austen, Emma, volume III, chapter 10

Thus, in rapid succession, passed the thoughts of Rosilia, who, endeavouring to assume placidity, once more essayed to express her thanks to Mrs. Melbourne for her kind attentions, […]

1838, [Letitia Elizabeth] Landon (indicated as editor), chapter XVI, in Duty and Inclination: […], volume III, London: Henry Colburn, […], page 207

The swift and indifferent placidity of that look troubled me.

1899 February, Joseph Conrad, “The Heart of Darkness”, in Blackwood’s Edinburgh Magazine, volume CLXV, number M, New York, N.Y.: The Leonard Scott Publishing Company, […], part I, page 200