Green’s Dictionary of Slang

1. the penis.

1966		Trimble 5000 Adult Sex Words and Phrases.
1978		L. Kramer Faggots 321: Peter, piccolo, piston, poker, pole, pork, prong, pud, roger, rupert.

2. (orig. milit.) a generic name for any young male aristocrat.

2001		Guardian G2 4: He thinks the SAS has become ‘top-heavy with officers’ — Ruperts, as he calls them.
2003		T. Pratchett Monstrous Regiment 73: ‘Who’s looking after the rupert?’ [...] The corporal sighed. ‘The officer. They ’re all called Rupert or Rodney or Tristram, or something’.
2004		(con. WWI) R. Holmes Tommy 307: Tom and his ilk [...] often apt at imitating [middle-class accents]. The practice was called ‘talking Rupert.’.