"Sit bolt upright" can be described using several synonyms. One term that can be used is "sit erect," which suggests a straight and upright position. Another synonym for this phrase is "sit up straight," which conveys how a person is sitting with their back straight and shoulders squared. Additionally, one can say "mount straight," which implies a position of attention and alertness. "Be at attention" is also a phrase that can be used to convey the meaning of being in a position of alertness and focus. Regardless of the term used, sitting bolt upright suggests a posture that is attentive and engaged.