Green’s Dictionary of Slang

1. (US campus) a diploma.

1896		W.C. Gore Student Sl. in Cohen (1997) 6: Dip [...] Diploma.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 32: dip, n. A diploma.

2. (Aus./UK/US) diphtheria.

1938		L. Ortzen Down Donkey Row 176: First they takes two of ’em to ’orspital wiv dip.
1941		V. Palmer ‘Josie’ in Mann Coast to Coast 2: It’s dip. The nurse said it was dip.

3. a member of the Diplomatic Service.

1960, 1975		Wentworth & Flexner DAS.
1968		‘John le Carré’ Small Town in Germany 111: A dip who graced your parlor.
2023		M. Herron Secret Hours 207: Dips were diplomats; lunching them part of his job.