Ab

The 1940 instructions told enumerators to list everyone whose usual home was with the household, including people away for a while. They wrote Ab after the name of a person temporarily absent, such as a traveling salesman or a student with sleeping quarters elsewhere.

So Ab means the person was counted with this household even though he or she was away when the enumerator called. It does not mean the person had left home for good, so expect to find the same name with the family in later records.