Aren't these all the same definitions, merely indicating an approximation?

"I'm about to arrive" -> "I'm close to arrival but not here right now".

"This book is about..." -> "This book is roughly this short approximation, because I can't fit the entire synopsis in a sentence,..."

"About his person" -> "Close to him, possibly so close as to be carrying it"

"He is about" -> "He is somewhere around here, but not in this exact spot"

"He is out and about" -> "He is out somewhere in this area"

"Gone walkabout" -> "Traveling with no specific destination"

"Roundabout" -> "Traffic control method that encourages vehicles to take a more circuitous passage, rather than directly crossing an intersection"

The context of the word changes the interpretation, but the essential function of "about" is the same in each case.