Having had a nightmare about "moneybags tonight"--the premonition for Shylock is serious for him, although comic to audiences--Shylock orders his daughter Jessica to remained locked in the house as the costumed and masked "Christian fools" revel in the street, ironically worried that they may rob him when it is his daughter who does so later. In an oath, Shylock also expresses his reluctance and distaste for going to dine with Antonio and Bassanio,
....By Jacob's staff I swear
I have no mind of feasting forth tonight,
But I will go. (2.5.36-38)
In the Old Testament and the Talmud, Jacob was the son of Isaac and the grandson of Abraham. Renamed "Israel" by God, Jacob is the forefather of the tribes of Israel (Jews), one of whose descendants is Shylock. Thus, Shylock's oath reminds the audience of his ethnicity as well as his allusion to the difficulties encountered by the Israelites, which he seems to suggest are similar to his own.
As further explanation, unlike his father Isaac, who lived rather harmoniously with his neighbors, Jacob experienced many personal conflicts such as the hatred of Esau, his brother, the deception of Laban, his father-in-law, the death of Rachel, his wife, even the rape of Dinah, his daughter. Because of all these hardships, the life of Jacob has come to represent the struggles of the Jewish people. Thus, Shylock's allusion to his ancestor Jacob illuminates his feelings of personal hardship and conflict that are also characteristic struggles of his people. Furthermore, his implication, then, in using "Jacob's staff" is that he, a Jew, being led by his ancestor who took the Israelites safely into Egypt,(but which later enslaved them) is wary of dining with the Christians, who are his enemy.