This is a term which is often used to express one's interest in a member of the female sex.

You would for example; say, "Mmm. She's a nice bit of crumpet." This would be considered both derogatory and chauvinistic. You would not have much success with the young woman in question, and would be branded a pig.

The term is thought to have its origins in a misplaced and ill-considered comparison between a young woman and a breakfast food. Though the similarity was never proved, the metaphor stuck.