This may look right to a Yahoo! Finance writer and editor, but according to most authorities, it’s at worst a misspelling and at best a variant of harebrained:
Even those authorities that consider it a variant don’t include a hyphen in hairbrained. Here’s what the American Heritage Dictionary says:
The first recorded use of harebrained dates to 1548. The spelling hairbrained also has a long history, going back to the 1500s when hair was a variant spelling of hare. The hair variant was preserved in Scotland into the 1700s, and as a result it is impossible to tell exactly when people began writing hairbrained in the belief that the word means “having a hair-sized brain” rather than “with no more sense than a hare.” While hairbrained continues to be used, the standard spelling of the word is harebrained.