The entry for Custis on page 397 of the
Dictionary of American Family Names (2003) written by Patrick Hanks says the following:
Custis (473) English: unexplained. Possibly from a much altered form of the personal name AUGUSTUS. This is an old VA surname, dating from the 17th century.
Link to the page in question:
https://books.google.de/books?id=FJoDDAAAQBAJ&pg=PA397 (in English).
Also, you should know that this website has a message board specifically dedicated to surnames, which is better suited for questions like yours. As such, you might perhaps have better luck there. Here it is:
https://surnames.behindthename.com/bb/sur/
I hope that helps! :)
"It is no measure of health to be well-adjusted to a profoundly sick society." ~ Jiddu Krishnamurti (1895-1986)