​Once-powerful politician Dean Skelos threw his beloved son under the bus Tuesday, telling ​jurors at their Manhattan corruption trial that it was “improper” of Adam ​Skelos ​to accept money for no-work ​jobs obtained using his lofty name — ​and that he had no idea it was happening.

Dean Skelos made the comment on his second day of cross-examination amid questioning over a $20,000 check Adam cashed for title insurance work he never did​.

Federal prosecutor Edward Diskant​ asked ​the former state Senate majority leader why he was “disappointed” in Adam for accepting the money without lifting a finger.

“The reason you were disappointed is because you thought it was improper​?” Diskant asked.

“I thought it was improper by him … and the people who gave him the check,” Skelos ​answered.

Later, Skelos also said he had been “surprised” to learn at trial that Adam had invoked his dad’s “reach” in a wiretapped call while trying to win business from a Greek diner owner.

“Senator, were you surprised to learn Adam was invoking your name in a call about his ‘reach’?” Diskant asked.

“Yes,” ​Skelos said.

“That had never happened before?” ​Diskant asked.​

“It may have​,​ but I wouldn’t approve it,”​ ​Skelos responded.

Skelos took the witness stand in his corruption retrial Friday and immediately copped to asking people ​who were ​lobbying ​before him ​to get his son jobs. But he denied he did it in exchange for help with legislation — claiming the bills passed for these companies were not controversial.

The Skeloses are on trial for pressuring companies lobbying Dean to give Adam do-nothing jobs and consulting gigs.

Dean Skelos did not take the stand at ​their first trial in 2015, which resulted in conviction​s that were overturned on appeal.

​Still, cross-examination ended Tuesday morning without any major cracks or slip-ups by the Long Island Republican.

But prosecutors urged the jury not to buy ​his denials of any knowledge of the alleged graft in closing arguments, which also started Tuesday, saying Dean Skelos was playing politician on the witness stand.

“For 30 years, Dean Skelos was in the business of convincing people to vote for him. And that is exactly what he was doing on the witness stand,” prosecutor Tom McKay told the jury.

“He is trying to win your vote — make you feel sympathy for him,” McKay said in closing arguments.

On Friday, Skelos choked up with emotion when discussing his troubled adopted son, who he claimed had all kinds of problems, including substance abuse issues and anger management. He told the jury that the only reason he had been asking people to help Adam was because his son was finally turning a corner after years of problems and he wanted to help him stay on track.

“A senator can’t use his power to make money for his son and say, ‘It’s OK because my son is having a hard time,'” McKay said. “It’s insulting to all of the people who go through these challenges and hardships in their life and don’t commit crimes.”