custom-variable-reset-standard is a compiled Lisp function in `cus-edit.el'.

(custom-variable-reset-standard WIDGET)

Restore standard setting for the variable edited by WIDGET.
This operation eliminates any saved setting for the variable,
restoring it to the state of a variable that has never been customized.
The value that was current before this operation
becomes the backup value, so you can get it again.