Charlotte Fedders, who was battered by her husband for 18 years, got another black eye last week from the courts.

A domestic relations master made a ruling that, if allowed to stand, could become a landmark of male chauvinist thinking, and an official invitation to wife-beaters to feel sorry for themselves.

Charlotte Fedders is the country's most celebrated victim of spouse abuse. Her husband was a high official of the Reagan administration, and her disclosures caused acute embarrassment to the family-oriented White House.

Fedders has written, with collaborator Laura Elliott, a book about her life as a punching bag for a husband who admits to being neurotic but blames her for baiting him. It is called "Shattered Dreams" and is being published by Harper and Row this week. John Fedders, one-time chief law enforcement officer of the Securities and Exchange Commission, went to court claiming that the book was about him, and that he was therefore entitled to share in the royalties.

Amazingly, the master who heard the case, John S. McInerney, agreed with him.

John Fedders is undisputably the person "without whom this book never could have been written" of many prefaces. If he had not beaten his wife, she would not have been able to write "Shattered Dreams." But the master's decision that he should be awarded 25 percent of the royalties goes beyond the unthinkable to the unbelievable.

By that standard, Adolf Hitler, had he lived, should have had a share in the proceeds from "The Diary of Anne Frank," the World War II memoir about a Jewish girl whose family was sheltered by a Dutch Christian family for 25 months. If Hitler had not ordered the extermination of Jews, she would not have had the material for her masterpiece.

The master was apparently much struck by the appearance of John Fedders as a witness. He told Charlotte Fedders' lawyer, Bryan Renehan, that Fedders was "exceptionally well-prepared."

Feminist groups, which are almost beside themselves over the decision, feel that while it is a financial setback for Charlotte Fedders, whose alimony payments were also reduced, it will advance their 15-year-old efforts to rouse the country on the issue of widespread wife-abuse. According to the National Coalition Against Domestic Violence, 3 million to 4 million women are battered every year in the U.S., one every 15 seconds.

Executive Director Marcia Niemann, herself once a battered wife, thinks the McInerney recommendations in the Fedders case is an extreme example of the tendency to "blame the victim" because it ratifies Fedders' claim that his wife is equally responsible for the ruination of their marriage.

Actually, McInerney seems to be echoing the "rule of thumb" of English common law, whereby a man was allowed to "chastise" his wife as long as he used a stick no wider than his thumb. His wife should have said "poor dear" when he had the blues. She didn't, and asked for what she got. The judge who first heard her suit for a limited divorce said John Fedders was guilty of "excessively vicious conduct and cruelty."

Fedders, who once earned $160,000 as a lawyer with the blue-chip firm of Arnold and Porter, does not deny he beat his wife. When she was pregnant with the first of their five sons, he punched her in the stomach.

Another time, when he found her in violation of his strict rule of no shoes in the house, he grabbed her by the hair and tried to throw her over a banister in the sight of the children.

But, says McInerney, it was half her fault. Many battered wives already feel that way, which is why they often stick it out so long.

Charlotte Fedders rose up on hearing Ronald Reagan's 1984 State of the Union address, in which he promised a drive against "horrible crimes like sexual abuse and family violence." She wrote a letter detailing her life of trying to stay alive although married. She never mailed it, but her sister gave a copy to the then-White House counsel Fred Fielding. The Wall Street Journal published the story and on Feb. 27, 1985, John Fedders resigned from the SEC.

Since then, Charlotte Fedders has been bringing up her children on her own. They all dig out at dawn every Thursday to distribute the Potomac Almanac. She also has been earning $5 an hour working in a flower shop.

In a bizarre moment of truth, John Fedders complained to the court that "the problem is that by hitting her, I gave her control over me."

He wants to make her pay for this outrageous reversal. The court, through McInerney, says she should. She will appeal.