What is perfidy (from Sister Carrie, by Theodore Dreiser)?

Perfidy is the deliberate breaking of faith; a betrayal of trust; treachery. In Sister Carrie, Charles Drouet and Carrie are live-in lovers. Carrie, however, is always on the lookout for something better and thinks that she has found it in George Hurstwood.

One night, Drouet begins to cross-examine Carrie about her relations with Hurstwood, revealing to her that Hurstwood is married:

Her little brain had been surging with contradictory feelings—shame at exposure, shame at Hurstwood's perfidy, anger at Drouet's deception, the mockery he had made at her. Now one clear idea came into her head. He (Drouet) was at fault.