Abbreviation for the name
Charles. Such shortenings were used mostly in
formal correspondence and documentation, prior to the
20th century.
Example: "The Hon. Chas. Thadwick, Esq."
Occasionally, a person might become known as "Chas" to his friends, through the virtue of having that particular string of letters plastered all over his things (at school, place of work, etc.), just as today, a person may go by his/her handle in "real life".
Also see Geo.