1
general::
Be weighed; also, be of a particular weight. For example, Because it was such a small plane, the passengers and their luggage had to weigh in before takeoff, or The fish weighed in at 18 pounds. [Late 1800s]
American Heritage Idioms
2
general::
Phrase(s): weigh in (at something)
Fig. to present oneself at a certain weight. (Usually said of boxers.) • The fighter weighed in at over two hundred pounds. • The contenders weighed in yesterday.
McGrawhill's American Idioms And Phrasal Verbs