Bonnie and Clyde made it quite clear how they felt about a former member of their gang in a letter they sent to him as he sat in the Dallas County Jail.

He was a coward, they wrote, and they should have killed him when they had the chance.

The four-page letter to Raymond Hamilton was written in April 1934 in Bonnie Parker's neat cursive and signed by Clyde Barrow. It could fetch more than $40,000 when it's sold next month by Boston-based RR Auction, said the auction house's executive vice president, Robert Livingston.

Based on the language, experts think Barrow, who had poor writing skills, likely dictated the letter to Parker, Livingston said Wednesday.

The couple was livid with Hamilton, in part because of a disagreement over how to split $4,000 stolen from a Texas bank just two months earlier.

"I should have killed you then I would have saved myself much bother and money looking for you," reads the letter, which is full of 1930s gangster jargon.

The letter also says Hamilton is "yellow," pointing out that he was captured without resistance and based on the way he acted during a narrow escape from a police road block in Missouri.

"The next impression was when we got the road 'blocked' on us in the Ozarks and you were too 'yellow' to fight. You cowered in the floorboard, afraid of being shot," the letter reads.

It also contains a prescient line about Bonnie and Clyde's own demise only a month later, when they were killed in a law enforcement ambush in Louisiana.

"I know that some day they will get me but it won't be without resistance," the letter reads.

It ends: "I hope this will serve the purpose of letting you know that you can never expect the least of sympathy or assistance from me. So long."

There is still intense interest in Bonnie and Clyde, who were almost glorified in their day before the public turned against them after they were linked to the murder of two police officers, Livingston said. Two pistols recovered from the bodies of the infamous outlaw couple were sold by RR in 2012 for more than $500,000.

It's unclear if Hamilton ever saw the letter, Livingston said. It was intercepted by Sheriff Richard "Smoot" Schmid, who shared it with newspapers several months later. It remained in his family's possession until they decided to auction it.

"Every line in this letter is remarkable," Livingston said. "It's one of a kind."