A cabaret-style dance move, popularized by the musical "
Fosse". Now used, usually ironically, to express excitement, glee, razzle dazzle, etc.
The move is performed by tilting the head slightly, shimmering the hands with fingers splayed either side of the face and crying "
Jazz hands!" with an enthusiastic smile. Think Jack from Will and Grace.
Also *
JAZZ HANDS!!!* used as an expressive punctuation on blogs and bulletin boards.