“our father Jacob” refers to the fact that the Samaritans claimed descent from Jacob through Joseph and his sons Ephraim and Mannaseh.

The people of the day had a great deal of reverence for their ancestors, even to the point of placing more authority to what they said than to what a contemporary would say.  Her reply indicates one of two things; either she was beginning to place more respect on Jesus or she was answering sarcastically because this stranger claims he can produce something better than her ancestors.  The latter seems much more likely and the irony of it is that the discrepancy between what Jesus can (and will) produce and what Jacob produced is infinitely great.