Lady Malvern was a hearty, happy, healthy, overpowering sort of dashed female, not so very tall but making up for it by measuring about six feet from the O.P. to the Prompt Side. She fitted into my biggest arm-chair as if it had been built round her by someone who knew they were wearing arm-chairs tight about the hips that season. She had bright, bulging eyes and a lot of yellow hair, and when she spoke she showed about fifty-seven front teeth. She was one of those women who kind of numb a fellow's faculties. She made me feel as if I were ten years old and had been brought into the drawing-room in my Sunday clothes to say how-d'you-do. Altogether by no means the sort of thing a chappie would wish to find in his sitting-room before breakfast.Apropos "measuring about six feet from the O.P. to the Prompt Side," if you'd been a fly on the wall when I had lunch with Peter Gill yesterday you would know that both “prompt side” and the abbreviation “O.P.” in the quote from the immortal Wodehouse above come from the theatrical stage.
On the UK stage cueing (triggering a lighting change, a sound effect, or some sort of stage or set movement) and prompting a forgotten line are done by the stage manager standing offstage in the wings. Traditionally, the prompting is done from the left side of the stage (as one faces the audience), also known as “stage left.” The abbreviation “O.P.” stands for “opposite prompt,” meaning the other side of the stage, i.e., “stage right.” Both terms date back at least to the 18th century.