To have very erect posture. I've never seen Olivia slouch—she's always as stiff as a ramrod.

Having a very erect posture, typically accompanied by a serious or formal demeanor. He sat ramrod straight at the table, glowering at me throughout the meal. She's always ramrod straight during class, studiously taking notes.

Having a very erect posture, typically accompanied by a serious or formal demeanor. He sat as straight as a ramrod at the table, glowering at me throughout the meal. She's always straight as a ramrod during class, studiously taking notes.

Very straight, rigid, or inflexible. Used to describe someone's physical posture or demeanor. The whole class sat in their chairs as stiff as ramrods during the principal's lecture. He stood as stiff as a ramrod after his name was called, too nervous to move.