Donna Motsinger's lawsuit against Bill Cosby is taking yet another dramatic turn ... Cosby wants to use evidence her son, Jeff, is an alcoholic with childhood trauma to undermine his testimony ... but Donna's lawyers say he can't do that!

In court papers, obtained by TMZ, Donna says that because Cosby has no expert to say that Jeff's "alcoholism or childhood trauma" is in any way related to Jeff's memory, any evidence of a drinking problem or history of trauma can't be used in the case to discredit his testimony.

As we reported ... Donna claims Cosby drugged and raped her in 1972 ... and pointed to Cosby admitting during a deposition that he obtained recreational prescriptions for Quaaludes that he secured from a gynecologist at a poker game.

Bill also said he planned to use the pills to give to women in the hopes of having sex with them.

Motsinger alleged Bill gave her a pill that she thought was aspirin. She claimed she felt off after taking it and said she woke up the next day in her bed with only her underwear on.

Remember ... Cosby served almost 3 years in prison for aggravated indecent assault but was released in 2021 after the Pennsylvania Supreme Court overturned his conviction based on a previous promise from a prosecutor not to charge him.