Ever since Pat Nixon, 73, suffered a severe stroke in 1976 and a second, milder, one in 1983, the state of her health has been a mystery to all but those in her intimate circle of friends. The reclusive former first lady never accompanies former president Richard Nixon on his domestic and foreign junkets, never appears at public events and never issues statements through a spokesman. She has rarely been seen.

Yet there she was last week, like any other shopper, browsing in the better dress department in New York City's Bonwit Teller. Looking for a silk dress ("All my life I've been allergic to wool," she told a sales clerk), Mrs. Nixon, accompanied by daughter Tricia, gave all the appearances of relishing her anonymity.

Even so, when a reporter from Washington recognized her she chatted easily and graciously about her own health ("I haven't been well, and I still get a little tired, but otherwise I feel fine") her grandchildren (Julie Nixon Eisenhower had her third child a year ago, she said; Tricia Nixon Cox's only child, Christopher, started kindergarten this fall) and clothes. When she noticed one long gown, she said it reminded her of the Alfred Bosand gowns she used to wear at the White House. But what she was really looking for was a coat, she said.

Mrs. Nixon spoke clearly and distinctly in her familiar, resonant voice with only a slight trace of slurring. She is still slender, still a strawberry blond and still a very private woman. She dismissed a request for an interview with a smile.

"I'm retired," she said with emphasis. "I was in politics, you know, for 28 years."

While Pat Nixon was out shopping, Richard Nixon and Republic of Korea's Prime Minister Lho Shin Yong were having lunch at New York's Lutece restaurant. This week, the South Korean leader is in Washington.

Last night Lho was guest of honor at a dinner cohosted by the Heritage Foundation's Edwin J. Feulner Jr. and former Reagan aide Richard V. Allen, who just returned from his 101st trip to Japan since 1967, when he was working for Nixon.

Among those invited were Assistant Secretary of State for East Asian and Pacific Affairs Paul D. Wolfowitz, who is expected to be named U.S. ambassador to Indonesia, and Gaston J. Sigur, currently senior special assistant for national security affairs, who is expected to be named Wolfowitz's successor.

Thanks to fellow National Security Council member George Bush, our man in Peking in 1974-75, national security affairs adviser Robert McFarlane has added this joke to his repertoire:

An angry Chinese father is asking his sons who kicked the outhouse in the Yangtze River. There is a kind of shuffling of feet and finally the No. 2 son holds up his hand and says, "I cannot tell a lie, father. I kicked outhouse in the Yangtze River."

The father is furious and thrashes the son within an ounce of his life. Begging for mercy, the son asks, "Father, you say George Washington tell truth and gets no punishment. What's the deal?"

Says the father: "George Washington's father not sitting in tree at the time."

Our source says the jokes are even better in dialect.

When billionaire oil tycoon Armand Hammer talked to Soviet leader Mikhail Gorbachev last June, he said he suggested that the Americans and Russians "test" each other's sincerity about peace.

"Gorbachev said, 'How do you mean test it?' " Hammer recalled at a recent State Department dinner. "I said, 'Well, first of all if you would both agree that neither one would be the first to use nuclear or conventional weapons, then we can sleep easy while negotiations are going on.'

"Secondly," Hammer continued, "I said, 'I know you're dead set against "Star Wars" but Reagan has said they're for defensive purposes only, that once we prove it works we'll share it with you.' I said, 'Why don't you suggest we share it right away?' "

According to Hammer, "Gorbachev liked both those ideas."

In the "Star Wars" goings-on closer to home: Hammer says President Reagan and Mrs. Reagan haven't invited him to their Nov. 9 White House dinner for Prince Charles and Princess Diana. However, Hammer is one of the 130 invited to the British Embassy dinner the following night.

Unusually reliable sources say that among the 80 names on the White House list are Walter and Leonore Annenberg, Betsy Bloomingdale, J. Carter and Pamela Brown, Jamie and Phyllis Wyeth and Peter Ustinov.

Buckingham Palace may deny it, but the word is that Ustinov was invited at Prince Charles' request.

Imagine the fix you'd be in if, in a fit of generosity, you had just lent your tiara to the National Gallery's "Treasure Houses of Britain" exhibit only to learn that the rest of the gang will be wearing theirs to the opening night lenders' dinner on Thursday.

What makes it even worse for the Marchioness of Tavistock is that she lent two tiaras, one a gold and pearl neoclassical style, circa 1808-1815; and the other of gold, silver and diamonds dating from the mid-19th century.

Wrote the Marquis of Tavistock, son of the Duke of Bedford, to National Gallery director J. Carter Brown: "I was amused you mentioned that people could wear tiaras if they wanted at the opening dinner since Henrietta's are in the show."

Proving that the Brits don't have a monopoly on royalty, Sweden's Prince Bertil and Princess Lilian are in town this week. They arrived without fanfare, though with plenty of fans.

Bertil delighted a National Press Club audience a few years ago when he told how he and Princess Lilian, a British-born divorce', lived together for 34 years but could not marry until his nephew, Sweden's King Carl Gustaf, came of age and was married. As second in line to the Swedish throne, Bertil could not have a divorced commoner for a wife.

Bertil and Lilian were at Georgetown University last night when Prof. Gary Hufbauer was introduced as the new Wallenberg Professor of International Finance and Diplomacy, a chair donated by the prominent Swedish banker Marcus Wallenberg.

Tonight, Swedish Ambassador Count Wilhelm Wachtmeister and Countess Ulla Wachtmeister are giving a dinner for the royals at the embassy. Vice President Bush's office confirms that he will be among the guests.

Entertaining everybody will be the U.S. Navy Band, which has written the guests into its bluegrass music routine. Countess Wachtmeister first heard the band on a Potomac River cruise aboard the barge of the chief of naval operations.