giggle-box
n.— «I can relate to this, as I was affectionately called “giggle-box” throughout my school years. It never took much to make me laugh; and, before too long everyone else in the class chimed in.» —“Jo Ellen Gluscevich” by Rx: Laughter Frederick News-Post (Maryland) May 10, 2007. (source: Double-Tongued Dictionary)