"Whereabout" is an adverb. It is considered obsolete, and is almost never used in modern English.
Definition: About which; concerning which.
Example: "I don't know the topic whereabout they were discussing."
"Whereabouts" can be used as an adverb or a noun.
Definition (Adverb): In, at, or near what location.
Example (Adverb): "Whereabouts do you live?"
Definition (Noun): Location; where something is situated.
Example (Noun): "The police are looking for the criminal, but they are unaware of his whereabouts."