NEW YORK — They sold in May and went away, all right.

With a disappointing finish on Thursday, the stock market closed what was by some measures its worst month in two years. Over five dismal weeks, Facebook fizzled, a debt crisis in Europe loomed, and nobody was in the mood to buy.

When May was mercifully over, the Dow Jones industrial average and other major indexes had erased most of the strong gains they built up through March and held on to in April.

“Any time the market dips like this, it erodes some confidence,” said Craig Callahan, co-founder and president of ICON Advisers in Denver. “It scares people out of the market. All of the above, May has done that.”

The Wall Street adage holds that investors should avoid the stock market for the months of May through October, commonly known as “sell in May and go away.”

It may not be sound strategy all the time — many financial advisers say it’s foolish — but this year it looked like good advice.

The Dow lost 820 points for the month, or 6.2 percent, its worst showing since May 2010. That month, investors were spooked by a one-day “flash crash” in stocks when a large trade overwhelmed computer servers.