ramrod straight

ˌramrod ˈstraight

,

(as) ˌstraight as a ˈramrod

(of a person) with a very straight back and looking serious and formal: As she walked in she could feel the tension in the room, with her mother sitting ramrod straight in her chair.
A ramrod is a long straight piece of iron used in the past to push explosive into a gun.
See also: ramrod, straight