His daddy's mother wasn't married when she gave birth. Her name was Schicklgruber (hence the derisive references to Adolf as "old Schicklgruber") and she later married a man named Heidler or Hitler . . . but Alois (Hitler's dad) didn't change his name to Heidler (or Hitler) until he was almost 40 years old.
There are long standing rumors that Maria Schiklgruber was working in a Jewish household when she got pregnant by the father or a son of the house . . . no documentary support but lots of questions. Our good friend Cecil Adams of The Straight Dope as usual has rounded up the info on this. I have always found Adams (or Ed Zotti if you prefer) to be surprisingly accurate (and not just for a column in an alternative newspaper - he has good researchers.)
Don't forget, Georg Heidler was probably Miss Schicklgruber's first cousin. What an inbred mess that family tree is!