In Act I Scene iii of The Merchant of Venice, explain the stanza, "When Jacob grazed his uncle Laban's sheep- This Jacob from our holy Abraham was, As his wise mother wrought in his behalf,...This was a way to thrive, and he was blest: And thrift is blessing , if men steal it not."
Shylock and Antonio have been negotiating the terms of the loan that Shylock will grant on Bassanio's behalf. He is somewhat surprised "Methoughts you said you neither lend nor borrow"(63) at Antonio's agreement to take the loan. He uses a biblical reference to make his point that "thrift is blessing" meaning that there is nothing wrong in being wise and making a profit as long as "men steal it not," in other words as long as it is honest business.
The reference itself, from The Book of Genesis, refers to Jacob, a descendent of Abram (who is important in both Jewish and Christian biblical history). Jacob's wise mother confirms ("wrought") that he is the third generation ("third possessor"). He cares for ("grazed") his uncle's sheep and wants to marry his uncle's daughter.
When Laban and Jacob come to an agreement - "were compromis'd"- and Jacob is to keep any sheep that is "streaked and pied," in others words with color streaks or a pattern of spots, Jacob cleverly arranges some sticks ("pill'd me certain wands) close by the sheep "between these woolly breeders in the act" and as they are mating or doing "the work of generation" he shows them to the "fulsome ewes" with the result that, at lambing (eaning") time, "parti-colored lambs" are born which therefore are Jacob's.
Shylock's point is that Jacob acts wisely and this renders him successful and that is "a blessing."