1
general::
Also, the in thing. Something in style or vogue, the latest trend. For example, Wearing oversized clothes is the thing these days, or Vans and sports utility vehicles have been the in thing for some time now. The first term dates from the mid-1700s; the second, which uses in in the sense of "popular," dates from the mid-1900s. Also see THE LATEST.
American Heritage Idioms