WATCH: Biographer Explains Why Eric Trump Is a 'Doofus' Following Trump Org. Testimony

Tim O'Brien, who has long written about the Trump family and their business dealings, told MSNBC on Thursday night that Eric Trump is a doofus. 

This week, Donald Trump's 3 oldest children have been called to testify in the New York fraud case. Donald Trump Jr. went first and answered the questions in a relatively genial manner. Eric Trump, who went second, handled things a bit differently. He was aggressive and angry. 

Lawrence O'Donnell asked O'Brien about Trump's contentions that he 'poured concrete' and didn't pay attention to the company's finances. "Eric Trump does not have callouses on his hand," the author responded. "Eric Trump has never labored in the hot sun on a highway making sure it’s smooth, the driveway going to Bedminster or wherever else he’s talking about.”

"Good job, Dad,” O’Brien said of Donald Trump. “You’ve led your children through life teaching them that they can try to get by on spin and they can try to get by on falsehoods but at the end of the day you’re going to be left alone like everyone around Donald Trump is because loyalty is a one-way street in the Trump universe and he’s even leaving his son hanging here."

The author closed, "[Eric is] the worst witness you could ever have. Even among his siblings [he] was routinely derided as being incapable and a bit of a doofus."