Marchbanks is a reference to a character for George Bernard Shaw's play Candida. In the book Looking Back in Anger, the characters are discussing Jimmy's romantic history, and he is called "Marchbanks" briefly. This is a reference to his romantic triste with an older woman. It is claimed, in the book, that Jimmy once dated a woman ten years his senior.

In Shaw's play, Marchbanks is a young man who ends up having an affair with an older woman, the wife of a clergyman, when he was just eighteen years old. Since Jimmy was that age when he was romantically involved with Madeleine, who, at the time, was twenty-eight years old, he is appropriately compared to the youthful Marchbanks having a relationship with an older woman.

It's a playful insult used by Cliff towards his friend Jimmy Porter. Eugene Marchbanks is the name of a character in Candida, a play by George Bernard Shaw. In the play, Marchbanks is a young, romantic poet who falls in love with an older woman, the wife of an Anglican clergyman.

In Look Back in Anger,everyone's having a discussion about Jimmy's former girlfriends. Alison brings up the name Madeline, which doesn't ring a bell to Cliff. Alison helpfully reminds Cliff that Madeline was Jimmy's mistress when he was fourteen. Jimmy quickly corrects her, saying that he was actually eighteen at the time of the affair. It is this interjection that prompts Cliff's little barb:

"CLIFF: I get mixed up with all your women. Was she the one all those years older than you? JIMMY: Ten years. CLIFF: Proper little Marchbanks, you are!"

In Candida, the character of Marchbanks is eighteen years old, just as Jimmy was when he became involved with Madeline. By comparing Jimmy to Marchbanks, Cliff is making a snide reference to his affair with an older woman.