It was far worse than a slip: Maidenform Brands Inc. lost a quarter of its market value, suffering the biggest plunge since the Iselin, N.J., company’s July 2005 initial public offering. The maker of women’s undergarments cut its full-year earnings forecast to $1.77 per share, at most, below the average analyst estimate of $2.17. Maidenform cited slowing consumer traffic and higher costs.