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.