in a position where you are sitting up with your back very straight.

He woke to see her sitting bolt upright beside him and wondered what was the matter.

• gas up something
• gather dust
• gather wits
• gather your wits
• on the run
• on the same lines
• on the same page
• on the same wavelength
• on the scene
• on the scrap heap
• on the shelf
• on the side
• on the sidelines
• on the side of the angels
• on the sly
• on-the-spot
• on the spot
• on the spur of the moment
• on the starting blocks
• on the strength of
• on the strength of something
• on the stump
• on the table
• on the threshold of
• on the threshold of something