Nancy Lady Keith Is Dead at 73; A Socialite Known for Her Wit

Nancy Lady Keith, a legendary beauty and participant in the social scene of New York and Hollywood, died of heart failure yesterday at New York Hospital. She was 73 years old and lived in Manhattan.

Lady Keith, known to her friends as Slim, had just completed her memoirs, ''Memories of a Rich and Imperfect Life,'' which is to be published later this year by Simon & Schuster.

Known for her wit and charm, she was a sought-after guest, a close friend of Ernest Hemingway, the Selznicks and much of Hollywood and New York society. Truman Capote, who called her ''Big Mama,'' was one of her closest friends until he wrote ''Answered Prayers,'' an article that caricatured New York society. After the article was published in Esquire she never spoke to Mr. Capote again.

In her later years her circle of friends included Mike Nichols and Jerome Robbins.

Married Hawks and Hayward

A native of Salinas, Calif., she acquired her title from her third husband, Lord Kenneth Keith, a British merchant banker, whom she married in 1962.

She was Mary Raye Gross at her birth on July 15, 1916, but her mother changed her name to Nancy. She grew up to be the kind of beauty that was sought after in Hollywood, although she never appeared in films. In her early 20's she became a regular guest at San Simeon, the Hearst estate, and in 1941 Gary Cooper gave her away when she married Howard Hawks, the director and producer. Their daughter, Kitty Hawks, survives her.

In 1946, 150 fashion editors and stylists named ''Slim'' Hawks the best-dressed woman of the year. The runner-up was the Duchess of Windsor. At the time, Mrs. Hawks was quoted as saying, ''I have a tall, skinny frame that clothes look well on. Seriously, just say that I'm a great believer in simplicity in clothes.''

In her Hollywood years she was credited with discovering and promoting such stars as Lauren Bacall, Joanne Dru and Ella Raines, all of whom resembled her somewhat in their casual, long-limbed type of good looks.

Two days after she and Mr. Hawks were divorced in 1949, she married Leland Hayward, the movie and theatrical producer, at the Manhasset, L.I., estate of William S. Paley. She and Mr. Hayward divorced in 1959 and three years later she married Kenneth Keith. They were divorced in 1972. ''I was married to each for about 10 years,'' she said at the time. ''I guess that's about all I can hack.''

In addition to her daughter, she is survived by a sister, Theodora Andersen, of Burlingame, Calif., and four stepchildren, Alastaire Keith and Camilla Mackesson, the children of Lord Keith, and William Hayward and Brooke Hayward Duchin.