Today it means: The practice of expressing a particular opinion or performing a particular action in an attempt to make oneself look virtuous: commonly defensively in response to, or anticipation of, criticism; or aggressively, in assuming a moral posture in attacking an opponent. Full definition →
Initially used in academic works about religion to refer to ostentatious public displays of religiosity. Popularised by James Bartholomew in an April 2015 article in the British conservative magazine The Spectator.
Etymology from Wiktionary, CC BY-SA 4.0.