Collocation 'bolt upright'
Like you said, it's an adverb.
bolt : ...(adverb) 26. Archaic. with sudden meeting or collision; suddenly.
Wiktionary defines:
bolt: suddenly; straight; unbendingly. e.g. "The soldiers stood bolt upright for inspection."
Like you said, it's an adverb.
bolt : ...(adverb) 26. Archaic. with sudden meeting or collision; suddenly.
Wiktionary defines:
bolt: suddenly; straight; unbendingly. e.g. "The soldiers stood bolt upright for inspection."