Green’s Dictionary of Slang

1. (US campus) a post-graduate.

1896		W.C. Gore Student Sl. in Cohen (1997) 7: p.g. n., a. Postgraduate.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 48: P.G. i.e. post-graduate n. A post-graduate student.

2. (US campus) a pretty girl.

1900

E.H. Babbitt ‘College Words and Phrases’ in DN II:i 48: P.G. i.e. [...] pretty girl, n. A pretty girl.

3. a paying guest.

1925		F. Stark 1 July in Moorehead Letters (1974) I 93: I am afraid I shall not be well enough to look after p.g.’s after all.
1938		S. Beckett Murphy (1963) 48: ‘A lady,’ said Murphy bitterly, ‘not a landlady [...] We are P.G.s.’.
1946		J. Cary Moonlight (1995) 123: I want to know if Aunty will take me in here, as a P.G. of course.
1965		G. Melly Owning Up (1974) 24: When I came to London to work I wrote and asked if they would have me as a P.G.