Green’s Dictionary of Slang

(orig. UK juv.) pious, always in a derog. sense of self-righteous, unctuous, poss. hypocritical.

1883		T.B. Reed Willoughby Captains (1887) 24: [He was] reputed ‘pi.’ as the more irreverent among the Willoughbites were wont to stigmatise any fellow who made a profession of goodness.
1891		Winchester Word-Book in DN IV: ii 136: He’s very pi now, he mugs all day.
1899		E.W. Hornung Amateur Cracksman (1992) 124: I found it devlish difficult to do without it after you turned pi!
1901		To-Day 22 Aug. 124, 2: The one blot on her staircase was an individual who... had turned ostentatiously pious. ‘I ’ates them pi-men, [...] as often as not it’s sheer ’ypocrisy’ [F&H].
1913		A. Lunn Harrovians 92: ‘Oh, he’s pi!’ said another with sour scorn.
1946		(con. 1912) B. Marshall George Brown’s Schooldays 149: Mr. Saracen told Daddy being pi’s just another way of being decent.