In Into the Ring episode we have heard that "Fisk had taken over Rigoletto's operation". Again in Rabbit in a Snowstorm episode we heard that "Did you hear about Rigoletto"
So exactly who is Rigoletto.
Don Rigoletto was a mafia boss who had various dealings with Fisk before and during the first season.
In the comics, Don Rigoletto was Kingpin's predecessor, and he employed Wilson Fisk as a bodyguard only to be killed by him in the end.
This answer contains spoilers for the first season of Daredevil.
We hear several references to Rigoletto in the first season of Daredevil, but it only becomes apparent why we are hearing his name so often (without ever actually seeing him) later on in the season.
We know about Rigoletto was a loan shark in Hell's Kitchen in the 1970's, who loaned money to Bill Fisk to fund Bill's campaign for City Council. After Bill lost the election, he was prepared to meet with Rigoletto to discuss repaying the loan - a meeting never happened, as;
Bill was killed by his son, Wilson Fisk.
We don't know a whole lot about Rigoletto after that point, but it is clear that he never stopped lending people money.
In the first episode of the season, "Into the Ring", Wesley (Wilson Fisk's right hand man) blackmails a police officer, who initial assumes Wesley was sent by Rigoletto, whom the police officer owes money to. Wesley explains that they (by which he means Wilson Fisk's operation) bought Rigoletto out, and that they now own his debts - and as such, own the police officer.
This turns out to be slightly untrue. As reporter Ben Ulrich learns after talking to a mafia boss fleeing the city in the third episode, "Rabbit in a Snowstorm";
Rigoletto was killed and then hacked to pieces. It is implied that his family suffered a similar fate.
It is likely that Wilson Fisk blames Rigoletto for the position his family was put in back in the 1970's, and as such blames Rigoletto for the murder of Bill Fisk.